本文介绍一些注意力机制的实现，包括MobileVITv1/MobileVITv2/DAT/CrossFormer/MOA。

【深度学习】注意力机制（一）

【深度学习】注意力机制（二）

【深度学习】注意力机制（三）

【深度学习】注意力机制（四）

【深度学习】注意力机制（五）

目录

一、MobileVITv1

二、MobileVITv2

三、DAT（Deformable Attention Transformer）

四、CrossFormer

五、MOA（multi-resolution overlapped attention）

---

一、MobileVITv1

论文地址：https://arxiv.org/pdf/2110.02178v2.pdf

如下图：

该代码块不能直接使用，有相关依赖，可以参考（代码来源）：

import math
from typing import Dict, Optional, Sequence, Tuple, Unionimport numpy as np
import torch
from torch import Tensor, nn
from torch.nn import functional as Ffrom cvnets.layers import ConvLayer2d, get_normalization_layer
from cvnets.modules.base_module import BaseModule
from cvnets.modules.transformer import LinearAttnFFN, TransformerEncoderclass MobileViTBlock(BaseModule):"""This class defines the `MobileViT block <https://arxiv.org/abs/2110.02178?context=cs.LG>`_Args:opts: command line argumentsin_channels (int): :math:`C_{in}` from an expected input of size :math:`(N, C_{in}, H, W)`transformer_dim (int): Input dimension to the transformer unitffn_dim (int): Dimension of the FFN blockn_transformer_blocks (Optional[int]): Number of transformer blocks. Default: 2head_dim (Optional[int]): Head dimension in the multi-head attention. Default: 32attn_dropout (Optional[float]): Dropout in multi-head attention. Default: 0.0dropout (Optional[float]): Dropout rate. Default: 0.0ffn_dropout (Optional[float]): Dropout between FFN layers in transformer. Default: 0.0patch_h (Optional[int]): Patch height for unfolding operation. Default: 8patch_w (Optional[int]): Patch width for unfolding operation. Default: 8transformer_norm_layer (Optional[str]): Normalization layer in the transformer block. Default: layer_normconv_ksize (Optional[int]): Kernel size to learn local representations in MobileViT block. Default: 3dilation (Optional[int]): Dilation rate in convolutions. Default: 1no_fusion (Optional[bool]): Do not combine the input and output feature maps. Default: False"""def __init__(self,opts,in_channels: int,transformer_dim: int,ffn_dim: int,n_transformer_blocks: Optional[int] = 2,head_dim: Optional[int] = 32,attn_dropout: Optional[float] = 0.0,dropout: Optional[int] = 0.0,ffn_dropout: Optional[int] = 0.0,patch_h: Optional[int] = 8,patch_w: Optional[int] = 8,transformer_norm_layer: Optional[str] = "layer_norm",conv_ksize: Optional[int] = 3,dilation: Optional[int] = 1,no_fusion: Optional[bool] = False,*args,**kwargs) -> None:conv_3x3_in = ConvLayer2d(opts=opts,in_channels=in_channels,out_channels=in_channels,kernel_size=conv_ksize,stride=1,use_norm=True,use_act=True,dilation=dilation,)conv_1x1_in = ConvLayer2d(opts=opts,in_channels=in_channels,out_channels=transformer_dim,kernel_size=1,stride=1,use_norm=False,use_act=False,)conv_1x1_out = ConvLayer2d(opts=opts,in_channels=transformer_dim,out_channels=in_channels,kernel_size=1,stride=1,use_norm=True,use_act=True,)conv_3x3_out = Noneif not no_fusion:conv_3x3_out = ConvLayer2d(opts=opts,in_channels=2 * in_channels,out_channels=in_channels,kernel_size=conv_ksize,stride=1,use_norm=True,use_act=True,)super().__init__()self.local_rep = nn.Sequential()self.local_rep.add_module(name="conv_3x3", module=conv_3x3_in)self.local_rep.add_module(name="conv_1x1", module=conv_1x1_in)assert transformer_dim % head_dim == 0num_heads = transformer_dim // head_dimglobal_rep = [TransformerEncoder(opts=opts,embed_dim=transformer_dim,ffn_latent_dim=ffn_dim,num_heads=num_heads,attn_dropout=attn_dropout,dropout=dropout,ffn_dropout=ffn_dropout,transformer_norm_layer=transformer_norm_layer,)for _ in range(n_transformer_blocks)]global_rep.append(get_normalization_layer(opts=opts,norm_type=transformer_norm_layer,num_features=transformer_dim,))self.global_rep = nn.Sequential(*global_rep)self.conv_proj = conv_1x1_outself.fusion = conv_3x3_outself.patch_h = patch_hself.patch_w = patch_wself.patch_area = self.patch_w * self.patch_hself.cnn_in_dim = in_channelsself.cnn_out_dim = transformer_dimself.n_heads = num_headsself.ffn_dim = ffn_dimself.dropout = dropoutself.attn_dropout = attn_dropoutself.ffn_dropout = ffn_dropoutself.dilation = dilationself.n_blocks = n_transformer_blocksself.conv_ksize = conv_ksizedef unfolding(self, feature_map: Tensor) -> Tuple[Tensor, Dict]:patch_w, patch_h = self.patch_w, self.patch_hpatch_area = int(patch_w * patch_h)batch_size, in_channels, orig_h, orig_w = feature_map.shapenew_h = int(math.ceil(orig_h / self.patch_h) * self.patch_h)new_w = int(math.ceil(orig_w / self.patch_w) * self.patch_w)interpolate = Falseif new_w != orig_w or new_h != orig_h:# Note: Padding can be done, but then it needs to be handled in attention function.feature_map = F.interpolate(feature_map, size=(new_h, new_w), mode="bilinear", align_corners=False)interpolate = True# number of patches along width and heightnum_patch_w = new_w // patch_w  # n_wnum_patch_h = new_h // patch_h  # n_hnum_patches = num_patch_h * num_patch_w  # N# [B, C, H, W] --> [B * C * n_h, p_h, n_w, p_w]reshaped_fm = feature_map.reshape(batch_size * in_channels * num_patch_h, patch_h, num_patch_w, patch_w)# [B * C * n_h, p_h, n_w, p_w] --> [B * C * n_h, n_w, p_h, p_w]transposed_fm = reshaped_fm.transpose(1, 2)# [B * C * n_h, n_w, p_h, p_w] --> [B, C, N, P] where P = p_h * p_w and N = n_h * n_wreshaped_fm = transposed_fm.reshape(batch_size, in_channels, num_patches, patch_area)# [B, C, N, P] --> [B, P, N, C]transposed_fm = reshaped_fm.transpose(1, 3)# [B, P, N, C] --> [BP, N, C]patches = transposed_fm.reshape(batch_size * patch_area, num_patches, -1)info_dict = {"orig_size": (orig_h, orig_w),"batch_size": batch_size,"interpolate": interpolate,"total_patches": num_patches,"num_patches_w": num_patch_w,"num_patches_h": num_patch_h,}return patches, info_dictdef folding(self, patches: Tensor, info_dict: Dict) -> Tensor:n_dim = patches.dim()assert n_dim == 3, "Tensor should be of shape BPxNxC. Got: {}".format(patches.shape)# [BP, N, C] --> [B, P, N, C]patches = patches.contiguous().view(info_dict["batch_size"], self.patch_area, info_dict["total_patches"], -1)batch_size, pixels, num_patches, channels = patches.size()num_patch_h = info_dict["num_patches_h"]num_patch_w = info_dict["num_patches_w"]# [B, P, N, C] --> [B, C, N, P]patches = patches.transpose(1, 3)# [B, C, N, P] --> [B*C*n_h, n_w, p_h, p_w]feature_map = patches.reshape(batch_size * channels * num_patch_h, num_patch_w, self.patch_h, self.patch_w)# [B*C*n_h, n_w, p_h, p_w] --> [B*C*n_h, p_h, n_w, p_w]feature_map = feature_map.transpose(1, 2)# [B*C*n_h, p_h, n_w, p_w] --> [B, C, H, W]feature_map = feature_map.reshape(batch_size, channels, num_patch_h * self.patch_h, num_patch_w * self.patch_w)if info_dict["interpolate"]:feature_map = F.interpolate(feature_map,size=info_dict["orig_size"],mode="bilinear",align_corners=False,)return feature_mapdef forward_spatial(self, x: Tensor) -> Tensor:res = xfm = self.local_rep(x)# convert feature map to patchespatches, info_dict = self.unfolding(fm)# learn global representationsfor transformer_layer in self.global_rep:patches = transformer_layer(patches)# [B x Patch x Patches x C] --> [B x C x Patches x Patch]fm = self.folding(patches=patches, info_dict=info_dict)fm = self.conv_proj(fm)if self.fusion is not None:fm = self.fusion(torch.cat((res, fm), dim=1))return fmdef forward_temporal(self, x: Tensor, x_prev: Optional[Tensor] = None) -> Union[Tensor, Tuple[Tensor, Tensor]]:res = xfm = self.local_rep(x)# convert feature map to patchespatches, info_dict = self.unfolding(fm)# learn global representationsfor global_layer in self.global_rep:if isinstance(global_layer, TransformerEncoder):patches = global_layer(x=patches, x_prev=x_prev)else:patches = global_layer(patches)# [B x Patch x Patches x C] --> [B x C x Patches x Patch]fm = self.folding(patches=patches, info_dict=info_dict)fm = self.conv_proj(fm)if self.fusion is not None:fm = self.fusion(torch.cat((res, fm), dim=1))return fm, patchesdef forward(self, x: Union[Tensor, Tuple[Tensor]], *args, **kwargs) -> Union[Tensor, Tuple[Tensor, Tensor]]:if isinstance(x, Tuple) and len(x) == 2:# for spatio-temporal MobileViTreturn self.forward_temporal(x=x[0], x_prev=x[1])elif isinstance(x, Tensor):# For image datareturn self.forward_spatial(x)else:raise NotImplementedError

二、MobileVITv2

论文地址：Separable Self-attention for Mobile Vision Transformers

如下图：

代码不可直接使用，可参考代码来源：

class MobileViTBlockv2(BaseModule):"""This class defines the `MobileViTv2 <https://arxiv.org/abs/2206.02680>`_ blockArgs:opts: command line argumentsin_channels (int): :math:`C_{in}` from an expected input of size :math:`(N, C_{in}, H, W)`attn_unit_dim (int): Input dimension to the attention unitffn_multiplier (int): Expand the input dimensions by this factor in FFN. Default is 2.n_attn_blocks (Optional[int]): Number of attention units. Default: 2attn_dropout (Optional[float]): Dropout in multi-head attention. Default: 0.0dropout (Optional[float]): Dropout rate. Default: 0.0ffn_dropout (Optional[float]): Dropout between FFN layers in transformer. Default: 0.0patch_h (Optional[int]): Patch height for unfolding operation. Default: 8patch_w (Optional[int]): Patch width for unfolding operation. Default: 8conv_ksize (Optional[int]): Kernel size to learn local representations in MobileViT block. Default: 3dilation (Optional[int]): Dilation rate in convolutions. Default: 1attn_norm_layer (Optional[str]): Normalization layer in the attention block. Default: layer_norm_2d"""def __init__(self,opts,in_channels: int,attn_unit_dim: int,ffn_multiplier: Optional[Union[Sequence[Union[int, float]], int, float]] = 2.0,n_attn_blocks: Optional[int] = 2,attn_dropout: Optional[float] = 0.0,dropout: Optional[float] = 0.0,ffn_dropout: Optional[float] = 0.0,patch_h: Optional[int] = 8,patch_w: Optional[int] = 8,conv_ksize: Optional[int] = 3,dilation: Optional[int] = 1,attn_norm_layer: Optional[str] = "layer_norm_2d",*args,**kwargs) -> None:cnn_out_dim = attn_unit_dimconv_3x3_in = ConvLayer2d(opts=opts,in_channels=in_channels,out_channels=in_channels,kernel_size=conv_ksize,stride=1,use_norm=True,use_act=True,dilation=dilation,groups=in_channels,)conv_1x1_in = ConvLayer2d(opts=opts,in_channels=in_channels,out_channels=cnn_out_dim,kernel_size=1,stride=1,use_norm=False,use_act=False,)super(MobileViTBlockv2, self).__init__()self.local_rep = nn.Sequential(conv_3x3_in, conv_1x1_in)self.global_rep, attn_unit_dim = self._build_attn_layer(opts=opts,d_model=attn_unit_dim,ffn_mult=ffn_multiplier,n_layers=n_attn_blocks,attn_dropout=attn_dropout,dropout=dropout,ffn_dropout=ffn_dropout,attn_norm_layer=attn_norm_layer,)self.conv_proj = ConvLayer2d(opts=opts,in_channels=cnn_out_dim,out_channels=in_channels,kernel_size=1,stride=1,use_norm=True,use_act=False,)self.patch_h = patch_hself.patch_w = patch_wself.patch_area = self.patch_w * self.patch_hself.cnn_in_dim = in_channelsself.cnn_out_dim = cnn_out_dimself.transformer_in_dim = attn_unit_dimself.dropout = dropoutself.attn_dropout = attn_dropoutself.ffn_dropout = ffn_dropoutself.n_blocks = n_attn_blocksself.conv_ksize = conv_ksizeself.enable_coreml_compatible_fn = getattr(opts, "common.enable_coreml_compatible_module", False)if self.enable_coreml_compatible_fn:# we set persistent to false so that these weights are not part of model's state_dictself.register_buffer(name="unfolding_weights",tensor=self._compute_unfolding_weights(),persistent=False,)def _compute_unfolding_weights(self) -> Tensor:# [P_h * P_w, P_h * P_w]weights = torch.eye(self.patch_h * self.patch_w, dtype=torch.float)# [P_h * P_w, P_h * P_w] --> [P_h * P_w, 1, P_h, P_w]weights = weights.reshape((self.patch_h * self.patch_w, 1, self.patch_h, self.patch_w))# [P_h * P_w, 1, P_h, P_w] --> [P_h * P_w * C, 1, P_h, P_w]weights = weights.repeat(self.cnn_out_dim, 1, 1, 1)return weightsdef _build_attn_layer(self,opts,d_model: int,ffn_mult: Union[Sequence, int, float],n_layers: int,attn_dropout: float,dropout: float,ffn_dropout: float,attn_norm_layer: str,*args,**kwargs) -> Tuple[nn.Module, int]:if isinstance(ffn_mult, Sequence) and len(ffn_mult) == 2:ffn_dims = (np.linspace(ffn_mult[0], ffn_mult[1], n_layers, dtype=float) * d_model)elif isinstance(ffn_mult, Sequence) and len(ffn_mult) == 1:ffn_dims = [ffn_mult[0] * d_model] * n_layerselif isinstance(ffn_mult, (int, float)):ffn_dims = [ffn_mult * d_model] * n_layerselse:raise NotImplementedError# ensure that dims are multiple of 16ffn_dims = [int((d // 16) * 16) for d in ffn_dims]global_rep = [LinearAttnFFN(opts=opts,embed_dim=d_model,ffn_latent_dim=ffn_dims[block_idx],attn_dropout=attn_dropout,dropout=dropout,ffn_dropout=ffn_dropout,norm_layer=attn_norm_layer,)for block_idx in range(n_layers)]global_rep.append(get_normalization_layer(opts=opts, norm_type=attn_norm_layer, num_features=d_model))return nn.Sequential(*global_rep), d_modeldef __repr__(self) -> str:repr_str = "{}(".format(self.__class__.__name__)repr_str += "\n\t Local representations"if isinstance(self.local_rep, nn.Sequential):for m in self.local_rep:repr_str += "\n\t\t {}".format(m)else:repr_str += "\n\t\t {}".format(self.local_rep)repr_str += "\n\t Global representations with patch size of {}x{}".format(self.patch_h,self.patch_w,)if isinstance(self.global_rep, nn.Sequential):for m in self.global_rep:repr_str += "\n\t\t {}".format(m)else:repr_str += "\n\t\t {}".format(self.global_rep)if isinstance(self.conv_proj, nn.Sequential):for m in self.conv_proj:repr_str += "\n\t\t {}".format(m)else:repr_str += "\n\t\t {}".format(self.conv_proj)repr_str += "\n)"return repr_strdef unfolding_pytorch(self, feature_map: Tensor) -> Tuple[Tensor, Tuple[int, int]]:batch_size, in_channels, img_h, img_w = feature_map.shape# [B, C, H, W] --> [B, C, P, N]patches = F.unfold(feature_map,kernel_size=(self.patch_h, self.patch_w),stride=(self.patch_h, self.patch_w),)patches = patches.reshape(batch_size, in_channels, self.patch_h * self.patch_w, -1)return patches, (img_h, img_w)def folding_pytorch(self, patches: Tensor, output_size: Tuple[int, int]) -> Tensor:batch_size, in_dim, patch_size, n_patches = patches.shape# [B, C, P, N]patches = patches.reshape(batch_size, in_dim * patch_size, n_patches)feature_map = F.fold(patches,output_size=output_size,kernel_size=(self.patch_h, self.patch_w),stride=(self.patch_h, self.patch_w),)return feature_mapdef unfolding_coreml(self, feature_map: Tensor) -> Tuple[Tensor, Tuple[int, int]]:# im2col is not implemented in Coreml, so here we hack its implementation using conv2d# we compute the weights# [B, C, H, W] --> [B, C, P, N]batch_size, in_channels, img_h, img_w = feature_map.shape#patches = F.conv2d(feature_map,self.unfolding_weights,bias=None,stride=(self.patch_h, self.patch_w),padding=0,dilation=1,groups=in_channels,)patches = patches.reshape(batch_size, in_channels, self.patch_h * self.patch_w, -1)return patches, (img_h, img_w)def folding_coreml(self, patches: Tensor, output_size: Tuple[int, int]) -> Tensor:# col2im is not supported on coreml, so tracing fails# We hack folding function via pixel_shuffle to enable coreml tracingbatch_size, in_dim, patch_size, n_patches = patches.shapen_patches_h = output_size[0] // self.patch_hn_patches_w = output_size[1] // self.patch_wfeature_map = patches.reshape(batch_size, in_dim * self.patch_h * self.patch_w, n_patches_h, n_patches_w)assert (self.patch_h == self.patch_w), "For Coreml, we need patch_h and patch_w are the same"feature_map = F.pixel_shuffle(feature_map, upscale_factor=self.patch_h)return feature_mapdef resize_input_if_needed(self, x):batch_size, in_channels, orig_h, orig_w = x.shapeif orig_h % self.patch_h != 0 or orig_w % self.patch_w != 0:new_h = int(math.ceil(orig_h / self.patch_h) * self.patch_h)new_w = int(math.ceil(orig_w / self.patch_w) * self.patch_w)x = F.interpolate(x, size=(new_h, new_w), mode="bilinear", align_corners=True)return xdef forward_spatial(self, x: Tensor, *args, **kwargs) -> Tensor:x = self.resize_input_if_needed(x)fm = self.local_rep(x)# convert feature map to patchesif self.enable_coreml_compatible_fn:patches, output_size = self.unfolding_coreml(fm)else:patches, output_size = self.unfolding_pytorch(fm)# learn global representations on all patchespatches = self.global_rep(patches)# [B x Patch x Patches x C] --> [B x C x Patches x Patch]if self.enable_coreml_compatible_fn:fm = self.folding_coreml(patches=patches, output_size=output_size)else:fm = self.folding_pytorch(patches=patches, output_size=output_size)fm = self.conv_proj(fm)return fmdef forward_temporal(self, x: Tensor, x_prev: Tensor, *args, **kwargs) -> Union[Tensor, Tuple[Tensor, Tensor]]:x = self.resize_input_if_needed(x)fm = self.local_rep(x)# convert feature map to patchesif self.enable_coreml_compatible_fn:patches, output_size = self.unfolding_coreml(fm)else:patches, output_size = self.unfolding_pytorch(fm)# learn global representationsfor global_layer in self.global_rep:if isinstance(global_layer, LinearAttnFFN):patches = global_layer(x=patches, x_prev=x_prev)else:patches = global_layer(patches)# [B x Patch x Patches x C] --> [B x C x Patches x Patch]if self.enable_coreml_compatible_fn:fm = self.folding_coreml(patches=patches, output_size=output_size)else:fm = self.folding_pytorch(patches=patches, output_size=output_size)fm = self.conv_proj(fm)return fm, patchesdef forward(self, x: Union[Tensor, Tuple[Tensor]], *args, **kwargs) -> Union[Tensor, Tuple[Tensor, Tensor]]:if isinstance(x, Tuple) and len(x) == 2:# for spatio-temporal data (e.g., videos)return self.forward_temporal(x=x[0], x_prev=x[1])elif isinstance(x, Tensor):# for image datareturn self.forward_spatial(x)else:raise NotImplementedError

三、DAT（Deformable Attention Transformer）

论文地址：Vision Transformer with Deformable Attention

如下图：

代码如下（代码来源）：

class DAttentionBaseline(nn.Module):def __init__(self, q_size, kv_size, n_heads, n_head_channels, n_groups,attn_drop, proj_drop, stride, offset_range_factor, use_pe, dwc_pe,no_off, fixed_pe, ksize, log_cpb):super().__init__()self.dwc_pe = dwc_peself.n_head_channels = n_head_channelsself.scale = self.n_head_channels ** -0.5self.n_heads = n_headsself.q_h, self.q_w = q_size# self.kv_h, self.kv_w = kv_sizeself.kv_h, self.kv_w = self.q_h // stride, self.q_w // strideself.nc = n_head_channels * n_headsself.n_groups = n_groupsself.n_group_channels = self.nc // self.n_groupsself.n_group_heads = self.n_heads // self.n_groupsself.use_pe = use_peself.fixed_pe = fixed_peself.no_off = no_offself.offset_range_factor = offset_range_factorself.ksize = ksizeself.log_cpb = log_cpbself.stride = stridekk = self.ksizepad_size = kk // 2 if kk != stride else 0self.conv_offset = nn.Sequential(nn.Conv2d(self.n_group_channels, self.n_group_channels, kk, stride, pad_size, groups=self.n_group_channels),LayerNormProxy(self.n_group_channels),nn.GELU(),nn.Conv2d(self.n_group_channels, 2, 1, 1, 0, bias=False))if self.no_off:for m in self.conv_offset.parameters():m.requires_grad_(False)self.proj_q = nn.Conv2d(self.nc, self.nc,kernel_size=1, stride=1, padding=0)self.proj_k = nn.Conv2d(self.nc, self.nc,kernel_size=1, stride=1, padding=0)self.proj_v = nn.Conv2d(self.nc, self.nc,kernel_size=1, stride=1, padding=0)self.proj_out = nn.Conv2d(self.nc, self.nc,kernel_size=1, stride=1, padding=0)self.proj_drop = nn.Dropout(proj_drop, inplace=True)self.attn_drop = nn.Dropout(attn_drop, inplace=True)if self.use_pe and not self.no_off:if self.dwc_pe:self.rpe_table = nn.Conv2d(self.nc, self.nc, kernel_size=3, stride=1, padding=1, groups=self.nc)elif self.fixed_pe:self.rpe_table = nn.Parameter(torch.zeros(self.n_heads, self.q_h * self.q_w, self.kv_h * self.kv_w))trunc_normal_(self.rpe_table, std=0.01)elif self.log_cpb:# Borrowed from Swin-V2self.rpe_table = nn.Sequential(nn.Linear(2, 32, bias=True),nn.ReLU(inplace=True),nn.Linear(32, self.n_group_heads, bias=False))else:self.rpe_table = nn.Parameter(torch.zeros(self.n_heads, self.q_h * 2 - 1, self.q_w * 2 - 1))trunc_normal_(self.rpe_table, std=0.01)else:self.rpe_table = None@torch.no_grad()def _get_ref_points(self, H_key, W_key, B, dtype, device):ref_y, ref_x = torch.meshgrid(torch.linspace(0.5, H_key - 0.5, H_key, dtype=dtype, device=device),torch.linspace(0.5, W_key - 0.5, W_key, dtype=dtype, device=device),indexing='ij')ref = torch.stack((ref_y, ref_x), -1)ref[..., 1].div_(W_key - 1.0).mul_(2.0).sub_(1.0)ref[..., 0].div_(H_key - 1.0).mul_(2.0).sub_(1.0)ref = ref[None, ...].expand(B * self.n_groups, -1, -1, -1) # B * g H W 2return ref@torch.no_grad()def _get_q_grid(self, H, W, B, dtype, device):ref_y, ref_x = torch.meshgrid(torch.arange(0, H, dtype=dtype, device=device),torch.arange(0, W, dtype=dtype, device=device),indexing='ij')ref = torch.stack((ref_y, ref_x), -1)ref[..., 1].div_(W - 1.0).mul_(2.0).sub_(1.0)ref[..., 0].div_(H - 1.0).mul_(2.0).sub_(1.0)ref = ref[None, ...].expand(B * self.n_groups, -1, -1, -1) # B * g H W 2return refdef forward(self, x):B, C, H, W = x.size()dtype, device = x.dtype, x.deviceq = self.proj_q(x)q_off = einops.rearrange(q, 'b (g c) h w -> (b g) c h w', g=self.n_groups, c=self.n_group_channels)offset = self.conv_offset(q_off).contiguous()  # B * g 2 Hg WgHk, Wk = offset.size(2), offset.size(3)n_sample = Hk * Wkif self.offset_range_factor >= 0 and not self.no_off:offset_range = torch.tensor([1.0 / (Hk - 1.0), 1.0 / (Wk - 1.0)], device=device).reshape(1, 2, 1, 1)offset = offset.tanh().mul(offset_range).mul(self.offset_range_factor)offset = einops.rearrange(offset, 'b p h w -> b h w p')reference = self._get_ref_points(Hk, Wk, B, dtype, device)if self.no_off:offset = offset.fill_(0.0)if self.offset_range_factor >= 0:pos = offset + referenceelse:pos = (offset + reference).clamp(-1., +1.)if self.no_off:x_sampled = F.avg_pool2d(x, kernel_size=self.stride, stride=self.stride)assert x_sampled.size(2) == Hk and x_sampled.size(3) == Wk, f"Size is {x_sampled.size()}"else:x_sampled = F.grid_sample(input=x.reshape(B * self.n_groups, self.n_group_channels, H, W), grid=pos[..., (1, 0)], # y, x -> x, ymode='bilinear', align_corners=True) # B * g, Cg, Hg, Wgx_sampled = x_sampled.reshape(B, C, 1, n_sample)q = q.reshape(B * self.n_heads, self.n_head_channels, H * W)k = self.proj_k(x_sampled).reshape(B * self.n_heads, self.n_head_channels, n_sample)v = self.proj_v(x_sampled).reshape(B * self.n_heads, self.n_head_channels, n_sample)attn = torch.einsum('b c m, b c n -> b m n', q, k) # B * h, HW, Nsattn = attn.mul(self.scale)if self.use_pe and (not self.no_off):if self.dwc_pe:residual_lepe = self.rpe_table(q.reshape(B, C, H, W)).reshape(B * self.n_heads, self.n_head_channels, H * W)elif self.fixed_pe:rpe_table = self.rpe_tableattn_bias = rpe_table[None, ...].expand(B, -1, -1, -1)attn = attn + attn_bias.reshape(B * self.n_heads, H * W, n_sample)elif self.log_cpb:q_grid = self._get_q_grid(H, W, B, dtype, device)displacement = (q_grid.reshape(B * self.n_groups, H * W, 2).unsqueeze(2) - pos.reshape(B * self.n_groups, n_sample, 2).unsqueeze(1)).mul(4.0) # d_y, d_x [-8, +8]displacement = torch.sign(displacement) * torch.log2(torch.abs(displacement) + 1.0) / np.log2(8.0)attn_bias = self.rpe_table(displacement) # B * g, H * W, n_sample, h_gattn = attn + einops.rearrange(attn_bias, 'b m n h -> (b h) m n', h=self.n_group_heads)else:rpe_table = self.rpe_tablerpe_bias = rpe_table[None, ...].expand(B, -1, -1, -1)q_grid = self._get_q_grid(H, W, B, dtype, device)displacement = (q_grid.reshape(B * self.n_groups, H * W, 2).unsqueeze(2) - pos.reshape(B * self.n_groups, n_sample, 2).unsqueeze(1)).mul(0.5)attn_bias = F.grid_sample(input=einops.rearrange(rpe_bias, 'b (g c) h w -> (b g) c h w', c=self.n_group_heads, g=self.n_groups),grid=displacement[..., (1, 0)],mode='bilinear', align_corners=True) # B * g, h_g, HW, Nsattn_bias = attn_bias.reshape(B * self.n_heads, H * W, n_sample)attn = attn + attn_biasattn = F.softmax(attn, dim=2)attn = self.attn_drop(attn)out = torch.einsum('b m n, b c n -> b c m', attn, v)if self.use_pe and self.dwc_pe:out = out + residual_lepeout = out.reshape(B, C, H, W)y = self.proj_drop(self.proj_out(out))return y, pos.reshape(B, self.n_groups, Hk, Wk, 2), reference.reshape(B, self.n_groups, Hk, Wk, 2)

四、CrossFormer

该论文有好几个模块论文地址：CROSSFORMER: A VERSATILE VISION TRANSFORMER HINGING ON CROSS-SCALE ATTENTION

SDA、LDA、DPB如下图：

网络结构如下图：

代码如下（代码来源）：

import torch
import torch.nn as nn
import torch.utils.checkpoint as checkpoint
from timm.models.layers import DropPath, to_2tuple, trunc_normal_class Mlp(nn.Module):def __init__(self, in_features, hidden_features=None, out_features=None, act_layer=nn.GELU, drop=0.):super().__init__()out_features = out_features or in_featureshidden_features = hidden_features or in_featuresself.fc1 = nn.Linear(in_features, hidden_features)self.act = act_layer()self.fc2 = nn.Linear(hidden_features, out_features)self.drop = nn.Dropout(drop)def forward(self, x):x = self.fc1(x)x = self.act(x)x = self.drop(x)x = self.fc2(x)x = self.drop(x)return xclass DynamicPosBias(nn.Module):def __init__(self, dim, num_heads, residual):super().__init__()self.residual = residualself.num_heads = num_headsself.pos_dim = dim // 4self.pos_proj = nn.Linear(2, self.pos_dim)self.pos1 = nn.Sequential(nn.LayerNorm(self.pos_dim),nn.ReLU(inplace=True),nn.Linear(self.pos_dim, self.pos_dim),)self.pos2 = nn.Sequential(nn.LayerNorm(self.pos_dim),nn.ReLU(inplace=True),nn.Linear(self.pos_dim, self.pos_dim))self.pos3 = nn.Sequential(nn.LayerNorm(self.pos_dim),nn.ReLU(inplace=True),nn.Linear(self.pos_dim, self.num_heads))def forward(self, biases):if self.residual:pos = self.pos_proj(biases) # 2Wh-1 * 2Ww-1, headspos = pos + self.pos1(pos)pos = pos + self.pos2(pos)pos = self.pos3(pos)else:pos = self.pos3(self.pos2(self.pos1(self.pos_proj(biases))))return posdef flops(self, N):flops = N * 2 * self.pos_dimflops += N * self.pos_dim * self.pos_dimflops += N * self.pos_dim * self.pos_dimflops += N * self.pos_dim * self.num_headsreturn flopsclass Attention(nn.Module):r""" Multi-head self attention module with dynamic position bias.Args:dim (int): Number of input channels.group_size (tuple[int]): The height and width of the group.num_heads (int): Number of attention heads.qkv_bias (bool, optional):  If True, add a learnable bias to query, key, value. Default: Trueqk_scale (float | None, optional): Override default qk scale of head_dim ** -0.5 if setattn_drop (float, optional): Dropout ratio of attention weight. Default: 0.0proj_drop (float, optional): Dropout ratio of output. Default: 0.0"""def __init__(self, dim, group_size, num_heads, qkv_bias=True, qk_scale=None, attn_drop=0., proj_drop=0.,position_bias=True):super().__init__()self.dim = dimself.group_size = group_size  # Wh, Wwself.num_heads = num_headshead_dim = dim // num_headsself.scale = qk_scale or head_dim ** -0.5self.position_bias = position_biasif position_bias:self.pos = DynamicPosBias(self.dim // 4, self.num_heads, residual=False)# generate mother-setposition_bias_h = torch.arange(1 - self.group_size[0], self.group_size[0])position_bias_w = torch.arange(1 - self.group_size[1], self.group_size[1])biases = torch.stack(torch.meshgrid([position_bias_h, position_bias_w]))  # 2, 2Wh-1, 2W2-1biases = biases.flatten(1).transpose(0, 1).float()self.register_buffer("biases", biases)# get pair-wise relative position index for each token inside the groupcoords_h = torch.arange(self.group_size[0])coords_w = torch.arange(self.group_size[1])coords = torch.stack(torch.meshgrid([coords_h, coords_w]))  # 2, Wh, Wwcoords_flatten = torch.flatten(coords, 1)  # 2, Wh*Wwrelative_coords = coords_flatten[:, :, None] - coords_flatten[:, None, :]  # 2, Wh*Ww, Wh*Wwrelative_coords = relative_coords.permute(1, 2, 0).contiguous()  # Wh*Ww, Wh*Ww, 2relative_coords[:, :, 0] += self.group_size[0] - 1  # shift to start from 0relative_coords[:, :, 1] += self.group_size[1] - 1relative_coords[:, :, 0] *= 2 * self.group_size[1] - 1relative_position_index = relative_coords.sum(-1)  # Wh*Ww, Wh*Wwself.register_buffer("relative_position_index", relative_position_index)self.qkv = nn.Linear(dim, dim * 3, bias=qkv_bias)self.attn_drop = nn.Dropout(attn_drop)self.proj = nn.Linear(dim, dim)self.proj_drop = nn.Dropout(proj_drop)self.softmax = nn.Softmax(dim=-1)def forward(self, x, mask=None):"""Args:x: input features with shape of (num_groups*B, N, C)mask: (0/-inf) mask with shape of (num_groups, Wh*Ww, Wh*Ww) or None"""B_, N, C = x.shapeqkv = self.qkv(x).reshape(B_, N, 3, self.num_heads, C // self.num_heads).permute(2, 0, 3, 1, 4)q, k, v = qkv[0], qkv[1], qkv[2]  # make torchscript happy (cannot use tensor as tuple)q = q * self.scaleattn = (q @ k.transpose(-2, -1))if self.position_bias:pos = self.pos(self.biases) # 2Wh-1 * 2Ww-1, heads# select position biasrelative_position_bias = pos[self.relative_position_index.view(-1)].view(self.group_size[0] * self.group_size[1], self.group_size[0] * self.group_size[1], -1)  # Wh*Ww,Wh*Ww,nHrelative_position_bias = relative_position_bias.permute(2, 0, 1).contiguous()  # nH, Wh*Ww, Wh*Wwattn = attn + relative_position_bias.unsqueeze(0)if mask is not None:nW = mask.shape[0]attn = attn.view(B_ // nW, nW, self.num_heads, N, N) + mask.unsqueeze(1).unsqueeze(0)attn = attn.view(-1, self.num_heads, N, N)attn = self.softmax(attn)else:attn = self.softmax(attn)attn = self.attn_drop(attn)x = (attn @ v).transpose(1, 2).reshape(B_, N, C)x = self.proj(x)x = self.proj_drop(x)return xdef extra_repr(self) -> str:return f'dim={self.dim}, group_size={self.group_size}, num_heads={self.num_heads}'def flops(self, N):# calculate flops for 1 group with token length of Nflops = 0# qkv = self.qkv(x)flops += N * self.dim * 3 * self.dim# attn = (q @ k.transpose(-2, -1))flops += self.num_heads * N * (self.dim // self.num_heads) * N#  x = (attn @ v)flops += self.num_heads * N * N * (self.dim // self.num_heads)# x = self.proj(x)flops += N * self.dim * self.dimif self.position_bias:flops += self.pos.flops(N)return flopsclass CrossFormerBlock(nn.Module):r""" CrossFormer Block.Args:dim (int): Number of input channels.input_resolution (tuple[int]): Input resulotion.num_heads (int): Number of attention heads.group_size (int): Group size.lsda_flag (int): use SDA or LDA, 0 for SDA and 1 for LDA.mlp_ratio (float): Ratio of mlp hidden dim to embedding dim.qkv_bias (bool, optional): If True, add a learnable bias to query, key, value. Default: Trueqk_scale (float | None, optional): Override default qk scale of head_dim ** -0.5 if set.drop (float, optional): Dropout rate. Default: 0.0attn_drop (float, optional): Attention dropout rate. Default: 0.0drop_path (float, optional): Stochastic depth rate. Default: 0.0act_layer (nn.Module, optional): Activation layer. Default: nn.GELUnorm_layer (nn.Module, optional): Normalization layer.  Default: nn.LayerNorm"""def __init__(self, dim, input_resolution, num_heads, group_size=7, lsda_flag=0,mlp_ratio=4., qkv_bias=True, qk_scale=None, drop=0., attn_drop=0., drop_path=0.,act_layer=nn.GELU, norm_layer=nn.LayerNorm, num_patch_size=1):super().__init__()self.dim = dimself.input_resolution = input_resolutionself.num_heads = num_headsself.group_size = group_sizeself.lsda_flag = lsda_flagself.mlp_ratio = mlp_ratioself.num_patch_size = num_patch_sizeif min(self.input_resolution) <= self.group_size:# if group size is larger than input resolution, we don't partition groupsself.lsda_flag = 0self.group_size = min(self.input_resolution)self.norm1 = norm_layer(dim)self.attn = Attention(dim, group_size=to_2tuple(self.group_size), num_heads=num_heads,qkv_bias=qkv_bias, qk_scale=qk_scale, attn_drop=attn_drop, proj_drop=drop,position_bias=True)self.drop_path = DropPath(drop_path) if drop_path > 0. else nn.Identity()self.norm2 = norm_layer(dim)mlp_hidden_dim = int(dim * mlp_ratio)self.mlp = Mlp(in_features=dim, hidden_features=mlp_hidden_dim, act_layer=act_layer, drop=drop)attn_mask = Noneself.register_buffer("attn_mask", attn_mask)def forward(self, x):H, W = self.input_resolutionB, L, C = x.shapeassert L == H * W, "input feature has wrong size %d, %d, %d" % (L, H, W)shortcut = xx = self.norm1(x)x = x.view(B, H, W, C)# group embeddingsG = self.group_sizeif self.lsda_flag == 0: # 0 for SDAx = x.reshape(B, H // G, G, W // G, G, C).permute(0, 1, 3, 2, 4, 5)else: # 1 for LDAx = x.reshape(B, G, H // G, G, W // G, C).permute(0, 2, 4, 1, 3, 5)x = x.reshape(B * H * W // G**2, G**2, C)# multi-head self-attentionx = self.attn(x, mask=self.attn_mask)  # nW*B, G*G, C# ungroup embeddingsx = x.reshape(B, H // G, W // G, G, G, C)if self.lsda_flag == 0:x = x.permute(0, 1, 3, 2, 4, 5).reshape(B, H, W, C)else:x = x.permute(0, 3, 1, 4, 2, 5).reshape(B, H, W, C)x = x.view(B, H * W, C)# FFNx = shortcut + self.drop_path(x)x = x + self.drop_path(self.mlp(self.norm2(x)))return xdef extra_repr(self) -> str:return f"dim={self.dim}, input_resolution={self.input_resolution}, num_heads={self.num_heads}, " \f"group_size={self.group_size}, lsda_flag={self.lsda_flag}, mlp_ratio={self.mlp_ratio}"def flops(self):flops = 0H, W = self.input_resolution# norm1flops += self.dim * H * W# LSDAnW = H * W / self.group_size / self.group_sizeflops += nW * self.attn.flops(self.group_size * self.group_size)# mlpflops += 2 * H * W * self.dim * self.dim * self.mlp_ratio# norm2flops += self.dim * H * Wreturn flopsclass PatchMerging(nn.Module):r""" Patch Merging Layer.Args:input_resolution (tuple[int]): Resolution of input feature.dim (int): Number of input channels.norm_layer (nn.Module, optional): Normalization layer.  Default: nn.LayerNorm"""def __init__(self, input_resolution, dim, norm_layer=nn.LayerNorm, patch_size=[2], num_input_patch_size=1):super().__init__()self.input_resolution = input_resolutionself.dim = dimself.reductions = nn.ModuleList()self.patch_size = patch_sizeself.norm = norm_layer(dim)for i, ps in enumerate(patch_size):if i == len(patch_size) - 1:out_dim = 2 * dim // 2 ** ielse:out_dim = 2 * dim // 2 ** (i + 1)stride = 2padding = (ps - stride) // 2self.reductions.append(nn.Conv2d(dim, out_dim, kernel_size=ps, stride=stride, padding=padding))def forward(self, x):"""x: B, H*W, C"""H, W = self.input_resolutionB, L, C = x.shapeassert L == H * W, "input feature has wrong size"assert H % 2 == 0 and W % 2 == 0, f"x size ({H}*{W}) are not even."x = self.norm(x)x = x.view(B, H, W, C).permute(0, 3, 1, 2)xs = []for i in range(len(self.reductions)):tmp_x = self.reductions[i](x).flatten(2).transpose(1, 2)xs.append(tmp_x)x = torch.cat(xs, dim=2)return xdef extra_repr(self) -> str:return f"input_resolution={self.input_resolution}, dim={self.dim}"def flops(self):H, W = self.input_resolutionflops = H * W * self.dimfor i, ps in enumerate(self.patch_size):if i == len(self.patch_size) - 1:out_dim = 2 * self.dim // 2 ** ielse:out_dim = 2 * self.dim // 2 ** (i + 1)flops += (H // 2) * (W // 2) * ps * ps * out_dim * self.dimreturn flopsclass Stage(nn.Module):""" CrossFormer blocks for one stage.Args:dim (int): Number of input channels.input_resolution (tuple[int]): Input resolution.depth (int): Number of blocks.num_heads (int): Number of attention heads.group_size (int): variable G in the paper, one group has GxG embeddingsmlp_ratio (float): Ratio of mlp hidden dim to embedding dim.qkv_bias (bool, optional): If True, add a learnable bias to query, key, value. Default: Trueqk_scale (float | None, optional): Override default qk scale of head_dim ** -0.5 if set.drop (float, optional): Dropout rate. Default: 0.0attn_drop (float, optional): Attention dropout rate. Default: 0.0drop_path (float | tuple[float], optional): Stochastic depth rate. Default: 0.0norm_layer (nn.Module, optional): Normalization layer. Default: nn.LayerNormdownsample (nn.Module | None, optional): Downsample layer at the end of the layer. Default: Noneuse_checkpoint (bool): Whether to use checkpointing to save memory. Default: False."""def __init__(self, dim, input_resolution, depth, num_heads, group_size,mlp_ratio=4., qkv_bias=True, qk_scale=None, drop=0., attn_drop=0.,drop_path=0., norm_layer=nn.LayerNorm, downsample=None, use_checkpoint=False,patch_size_end=[4], num_patch_size=None):super().__init__()self.dim = dimself.input_resolution = input_resolutionself.depth = depthself.use_checkpoint = use_checkpoint# build blocksself.blocks = nn.ModuleList()for i in range(depth):lsda_flag = 0 if (i % 2 == 0) else 1self.blocks.append(CrossFormerBlock(dim=dim, input_resolution=input_resolution,num_heads=num_heads, group_size=group_size,lsda_flag=lsda_flag,mlp_ratio=mlp_ratio,qkv_bias=qkv_bias, qk_scale=qk_scale,drop=drop, attn_drop=attn_drop,drop_path=drop_path[i] if isinstance(drop_path, list) else drop_path,norm_layer=norm_layer,num_patch_size=num_patch_size))# patch merging layerif downsample is not None:self.downsample = downsample(input_resolution, dim=dim, norm_layer=norm_layer, patch_size=patch_size_end, num_input_patch_size=num_patch_size)else:self.downsample = Nonedef forward(self, x):for blk in self.blocks:if self.use_checkpoint:x = checkpoint.checkpoint(blk, x)else:x = blk(x)if self.downsample is not None:x = self.downsample(x)return xdef extra_repr(self) -> str:return f"dim={self.dim}, input_resolution={self.input_resolution}, depth={self.depth}"def flops(self):flops = 0for blk in self.blocks:flops += blk.flops()if self.downsample is not None:flops += self.downsample.flops()return flopsclass PatchEmbed(nn.Module):r""" Image to Patch EmbeddingArgs:img_size (int): Image size.  Default: 224.patch_size (int): Patch token size. Default: [4].in_chans (int): Number of input image channels. Default: 3.embed_dim (int): Number of linear projection output channels. Default: 96.norm_layer (nn.Module, optional): Normalization layer. Default: None"""def __init__(self, img_size=224, patch_size=[4], in_chans=3, embed_dim=96, norm_layer=None):super().__init__()img_size = to_2tuple(img_size)# patch_size = to_2tuple(patch_size)patches_resolution = [img_size[0] // patch_size[0], img_size[0] // patch_size[0]]self.img_size = img_sizeself.patch_size = patch_sizeself.patches_resolution = patches_resolutionself.num_patches = patches_resolution[0] * patches_resolution[1]self.in_chans = in_chansself.embed_dim = embed_dimself.projs = nn.ModuleList()for i, ps in enumerate(patch_size):if i == len(patch_size) - 1:dim = embed_dim // 2 ** ielse:dim = embed_dim // 2 ** (i + 1)stride = patch_size[0]padding = (ps - patch_size[0]) // 2self.projs.append(nn.Conv2d(in_chans, dim, kernel_size=ps, stride=stride, padding=padding))if norm_layer is not None:self.norm = norm_layer(embed_dim)else:self.norm = Nonedef forward(self, x):B, C, H, W = x.shape# FIXME look at relaxing size constraintsassert H == self.img_size[0] and W == self.img_size[1], \f"Input image size ({H}*{W}) doesn't match model ({self.img_size[0]}*{self.img_size[1]})."xs = []for i in range(len(self.projs)):tx = self.projs[i](x).flatten(2).transpose(1, 2)xs.append(tx)  # B Ph*Pw Cx = torch.cat(xs, dim=2)if self.norm is not None:x = self.norm(x)return xdef flops(self):Ho, Wo = self.patches_resolutionflops = 0for i, ps in enumerate(self.patch_size):if i == len(self.patch_size) - 1:dim = self.embed_dim // 2 ** ielse:dim = self.embed_dim // 2 ** (i + 1)flops += Ho * Wo * dim * self.in_chans * (self.patch_size[i] * self.patch_size[i])if self.norm is not None:flops += Ho * Wo * self.embed_dimreturn flopsclass CrossFormer(nn.Module):r""" CrossFormerA PyTorch impl of : `CrossFormer: A Versatile Vision Transformer Based on Cross-scale Attention`  -Args:img_size (int | tuple(int)): Input image size. Default 224patch_size (int | tuple(int)): Patch size. Default: 4in_chans (int): Number of input image channels. Default: 3num_classes (int): Number of classes for classification head. Default: 1000embed_dim (int): Patch embedding dimension. Default: 96depths (tuple(int)): Depth of each stage.num_heads (tuple(int)): Number of attention heads in different layers.group_size (int): Group size. Default: 7mlp_ratio (float): Ratio of mlp hidden dim to embedding dim. Default: 4qkv_bias (bool): If True, add a learnable bias to query, key, value. Default: Trueqk_scale (float): Override default qk scale of head_dim ** -0.5 if set. Default: Nonedrop_rate (float): Dropout rate. Default: 0attn_drop_rate (float): Attention dropout rate. Default: 0drop_path_rate (float): Stochastic depth rate. Default: 0.1norm_layer (nn.Module): Normalization layer. Default: nn.LayerNorm.ape (bool): If True, add absolute position embedding to the patch embedding. Default: Falsepatch_norm (bool): If True, add normalization after patch embedding. Default: Trueuse_checkpoint (bool): Whether to use checkpointing to save memory. Default: False"""def __init__(self, img_size=224, patch_size=[4], in_chans=3, num_classes=1000,embed_dim=96, depths=[2, 2, 6, 2], num_heads=[3, 6, 12, 24],group_size=7, mlp_ratio=4., qkv_bias=True, qk_scale=None,drop_rate=0., attn_drop_rate=0., drop_path_rate=0.1,norm_layer=nn.LayerNorm, ape=False, patch_norm=True,use_checkpoint=False, merge_size=[[2], [2], [2]], **kwargs):super().__init__()self.num_classes = num_classesself.num_layers = len(depths)self.embed_dim = embed_dimself.ape = apeself.patch_norm = patch_normself.num_features = int(embed_dim * 2 ** (self.num_layers - 1))self.mlp_ratio = mlp_ratio# split image into non-overlapping patchesself.patch_embed = PatchEmbed(img_size=img_size, patch_size=patch_size, in_chans=in_chans, embed_dim=embed_dim,norm_layer=norm_layer if self.patch_norm else None)num_patches = self.patch_embed.num_patchespatches_resolution = self.patch_embed.patches_resolutionself.patches_resolution = patches_resolution# absolute position embeddingif self.ape:self.absolute_pos_embed = nn.Parameter(torch.zeros(1, num_patches, embed_dim))trunc_normal_(self.absolute_pos_embed, std=.02)self.pos_drop = nn.Dropout(p=drop_rate)# stochastic depthdpr = [x.item() for x in torch.linspace(0, drop_path_rate, sum(depths))]  # stochastic depth decay rule# build layersself.layers = nn.ModuleList()num_patch_sizes = [len(patch_size)] + [len(m) for m in merge_size]for i_layer in range(self.num_layers):patch_size_end = merge_size[i_layer] if i_layer < self.num_layers - 1 else Nonenum_patch_size = num_patch_sizes[i_layer]layer = Stage(dim=int(embed_dim * 2 ** i_layer),input_resolution=(patches_resolution[0] // (2 ** i_layer),patches_resolution[1] // (2 ** i_layer)),depth=depths[i_layer],num_heads=num_heads[i_layer],group_size=group_size[i_layer],mlp_ratio=self.mlp_ratio,qkv_bias=qkv_bias, qk_scale=qk_scale,drop=drop_rate, attn_drop=attn_drop_rate,drop_path=dpr[sum(depths[:i_layer]):sum(depths[:i_layer + 1])],norm_layer=norm_layer,downsample=PatchMerging if (i_layer < self.num_layers - 1) else None,use_checkpoint=use_checkpoint,patch_size_end=patch_size_end,num_patch_size=num_patch_size)self.layers.append(layer)self.norm = norm_layer(self.num_features)self.avgpool = nn.AdaptiveAvgPool1d(1)self.head = nn.Linear(self.num_features, num_classes) if num_classes > 0 else nn.Identity()self.apply(self._init_weights)def _init_weights(self, m):if isinstance(m, nn.Linear):trunc_normal_(m.weight, std=.02)if isinstance(m, nn.Linear) and m.bias is not None:nn.init.constant_(m.bias, 0)elif isinstance(m, nn.LayerNorm):nn.init.constant_(m.bias, 0)nn.init.constant_(m.weight, 1.0)@torch.jit.ignoredef no_weight_decay(self):return {'absolute_pos_embed'}@torch.jit.ignoredef no_weight_decay_keywords(self):return {'relative_position_bias_table'}def forward_features(self, x):x = self.patch_embed(x)if self.ape:x = x + self.absolute_pos_embedx = self.pos_drop(x)for layer in self.layers:x = layer(x)x = self.norm(x)  # B L Cx = self.avgpool(x.transpose(1, 2))  # B C 1x = torch.flatten(x, 1)return xdef forward(self, x):x = self.forward_features(x)x = self.head(x)return xdef flops(self):flops = 0flops += self.patch_embed.flops()for i, layer in enumerate(self.layers):flops += layer.flops()flops += self.num_features * self.patches_resolution[0] * self.patches_resolution[1] // (2 ** self.num_layers)flops += self.num_features * self.num_classesreturn flops

五、MOA（multi-resolution overlapped attention）

论文地址：Aggregating Global Features into Local Vision Transformer

如下图：

代码如下（代码来源）：

# --------------------------------------------------------
# Adopted from Swin Transformer
# Modified by Krushi Patel
# --------------------------------------------------------import torch
import torch.nn as nn
import torch.utils.checkpoint as checkpoint
from timm.models.layers import DropPath, to_2tuple, trunc_normal_
from einops.layers.torch import Rearrange, Reduceclass Mlp(nn.Module):def __init__(self, in_features, hidden_features=None, out_features=None, act_layer=nn.GELU, drop=0.):super().__init__()out_features = out_features or in_featureshidden_features = hidden_features or in_featuresself.fc1 = nn.Linear(in_features, hidden_features)self.act = act_layer()self.fc2 = nn.Linear(hidden_features, out_features)self.drop = nn.Dropout(drop)def forward(self, x):x = self.fc1(x)x = self.act(x)x = self.drop(x)x = self.fc2(x)x = self.drop(x)return xdef window_partition(x, window_size):"""Args:x: (B, H, W, C)window_size (int): window sizeReturns:windows: (num_windows*B, window_size, window_size, C)"""B, H, W, C = x.shapex = x.view(B, H // window_size, window_size, W // window_size, window_size, C)windows = x.permute(0, 1, 3, 2, 4, 5).contiguous().view(-1, window_size, window_size, C)return windowsdef window_reverse(windows, window_size, H, W):"""Args:windows: (num_windows*B, window_size, window_size, C)window_size (int): Window sizeH (int): Height of imageW (int): Width of imageReturns:x: (B, H, W, C)"""B = int(windows.shape[0] / (H * W / window_size / window_size))x = windows.view(B, H // window_size, W // window_size, window_size, window_size, -1)x = x.permute(0, 1, 3, 2, 4, 5).contiguous().view(B, H, W, -1)return xclass WindowAttention(nn.Module):r""" Window based multi-head self attention (W-MSA) module with relative position bias.It supports both of shifted and non-shifted window.Args:dim (int): Number of input channels.window_size (tuple[int]): The height and width of the window.num_heads (int): Number of attention heads.qkv_bias (bool, optional):  If True, add a learnable bias to query, key, value. Default: Trueqk_scale (float | None, optional): Override default qk scale of head_dim ** -0.5 if setattn_drop (float, optional): Dropout ratio of attention weight. Default: 0.0proj_drop (float, optional): Dropout ratio of output. Default: 0.0"""def __init__(self, dim, window_size, num_heads, qkv_bias=True, qk_scale=None, attn_drop=0., proj_drop=0.):super().__init__()self.dim = dimself.window_size = window_size  # Wh, Wwself.query_size = self.window_sizeself.key_size = self.window_size[0] * 2self.num_heads = num_headshead_dim = dim // num_headsself.scale = qk_scale or head_dim ** -0.5# define a parameter table of relative position biasself.relative_position_bias_table = nn.Parameter(torch.zeros((2 * window_size[0] - 1) * (2 * window_size[1] - 1), num_heads))  # 2*Wh-1 * 2*Ww-1, nH# get pair-wise relative position index for each token inside the windowcoords_h = torch.arange(self.window_size[0])coords_w = torch.arange(self.window_size[1])coords = torch.stack(torch.meshgrid([coords_h, coords_w]))  # 2, Wh, Wwcoords_flatten = torch.flatten(coords, 1)  # 2, Wh*Wwrelative_coords = coords_flatten[:, :, None] - coords_flatten[:, None, :]  # 2, Wh*Ww, Wh*Wwrelative_coords = relative_coords.permute(1, 2, 0).contiguous()  # Wh*Ww, Wh*Ww, 2relative_coords[:, :, 0] += self.window_size[0] - 1  # shift to start from 0relative_coords[:, :, 1] += self.window_size[1] - 1relative_coords[:, :, 0] *= 2 * self.window_size[1] - 1relative_position_index = relative_coords.sum(-1)  # Wh*Ww, Wh*Wwself.register_buffer("relative_position_index", relative_position_index)self.qkv = nn.Linear(dim, dim * 3, bias=qkv_bias)self.attn_drop = nn.Dropout(attn_drop)self.proj = nn.Linear(dim, dim)self.proj_drop = nn.Dropout(proj_drop)trunc_normal_(self.relative_position_bias_table, std=.02)self.softmax = nn.Softmax(dim=-1)def forward(self, x):"""Args:x: input features with shape of (num_windows*B, N, C)mask: (0/-inf) mask with shape of (num_windows, Wh*Ww, Wh*Ww) or None"""B_, N, C = x.shapeqkv = self.qkv(x).reshape(B_, N, 3, self.num_heads, C // self.num_heads).permute(2, 0, 3, 1, 4)q, k, v = qkv[0], qkv[1], qkv[2]  # make torchscript happy (cannot use tensor as tuple)q = q * self.scaleattn = (q @ k.transpose(-2, -1))relative_position_bias = self.relative_position_bias_table[self.relative_position_index.view(-1)].view(self.window_size[0] * self.window_size[1], self.window_size[0] * self.window_size[1], -1)  # Wh*Ww,Wh*Ww,nHrelative_position_bias = relative_position_bias.permute(2, 0, 1).contiguous()  # nH, Wh*Ww, Wh*Wwattn = attn + relative_position_bias.unsqueeze(0)attn = self.softmax(attn)attn = self.attn_drop(attn)x = (attn @ v).transpose(1, 2).reshape(B_, N, C)x = self.proj(x)x = self.proj_drop(x)return xdef extra_repr(self) -> str:#return f'dim={self.dim}, window_size={self.window_size}, num_heads={self.num_heads}'return f'dim={self.dim}, num_heads={self.num_heads}'def flops(self, N):# calculate flops for 1 window with token length of Nflops = 0# qkv = self.qkv(x)flops += N * self.dim * 3 * self.dim# attn = (q @ k.transpose(-2, -1))flops += self.num_heads * N * (self.dim // self.num_heads) * N#  x = (attn @ v)flops += self.num_heads * N * N * (self.dim // self.num_heads)# x = self.proj(x)flops += N * self.dim * self.dimreturn flopsclass GlobalAttention(nn.Module):r""" MOA - multi-head self attention (W-MSA) module with relative position bias.Args:dim (int): Number of input channels.window_size (tuple[int]): The height and width of the window.num_heads (int): Number of attention heads.qkv_bias (bool, optional):  If True, add a learnable bias to query, key, value. Default: Trueqk_scale (float | None, optional): Override default qk scale of head_dim ** -0.5 if setattn_drop (float, optional): Dropout ratio of attention weight. Default: 0.0proj_drop (float, optional): Dropout ratio of output. Default: 0.0"""def __init__(self, dim, window_size, input_resolution,num_heads, qkv_bias=True, qk_scale=None, attn_drop=0., proj_drop=0.):super().__init__()self.dim = dimself.window_size = window_size  # Wh, Wwself.query_size = self.window_size[0]self.key_size = self.window_size[0] + 2h,w = input_resolutionself.seq_len = h//self.query_sizeself.num_heads = num_headshead_dim = dim // num_headsself.scale = qk_scale or head_dim ** -0.5self.reduction = 32self.pre_conv = nn.Conv2d(dim, int(dim//self.reduction), 1)# define a parameter table of relative position biasself.relative_position_bias_table = nn.Parameter(torch.zeros((2 * self.seq_len - 1) * (2 * self.seq_len - 1), num_heads))  # 2*Wh-1 * 2*Ww-1, nH#print(self.relative_position_bias_table.shape)# get pair-wise relative position index for each token inside the windowcoords_h = torch.arange(self.seq_len)coords_w = torch.arange(self.seq_len)coords = torch.stack(torch.meshgrid([coords_h, coords_w]))  # 2, Wh, Wwcoords_flatten = torch.flatten(coords, 1)  # 2, Wh*Wwrelative_coords = coords_flatten[:, :, None] - coords_flatten[:, None, :]  # 2, Wh*Ww, Wh*Wwrelative_coords = relative_coords.permute(1, 2, 0).contiguous()  # Wh*Ww, Wh*Ww, 2relative_coords[:, :, 0] += self.seq_len - 1  # shift to start from 0relative_coords[:, :, 1] += self.seq_len - 1relative_coords[:, :, 0] *= 2 * self.seq_len - 1relative_position_index = relative_coords.sum(-1)  # Wh*Ww, Wh*Wwself.register_buffer("relative_position_index", relative_position_index)self.queryembedding = Rearrange('b c (h p1) (w p2) -> b (p1 p2 c) h w', p1 = self.query_size, p2 = self. query_size)self.keyembedding = nn.Unfold(kernel_size=(self.key_size, self.key_size), stride = 14, padding=1)self.query_dim = int(dim//self.reduction) * self.query_size * self.query_sizeself.key_dim = int(dim//self.reduction) * self.key_size * self.key_sizeself.q = nn.Linear(self.query_dim, self.dim,bias=qkv_bias)self.kv = nn.Linear(self.key_dim, 2*self.dim,bias=qkv_bias)self.attn_drop = nn.Dropout(attn_drop)self.proj = nn.Linear(dim,dim)self.proj_drop = nn.Dropout(proj_drop)#trunc_normal_(self.relative_position_bias_table, std=.02)self.softmax = nn.Softmax(dim=-1)def forward(self, x, H, W):"""Args:x: input features with shape of (num_windows*B, N, C)mask: (0/-inf) mask with shape of (num_windows, Wh*Ww, Wh*Ww) or None"""#B, H, W, C = x.shapeB,_, C = x.shape  x = x.reshape(-1, C, H, W)    x = self.pre_conv(x)query = self.queryembedding(x).view(B,-1,self.query_dim)query = self.q(query)B,N,C = query.size()q = query.reshape(B,N,self.num_heads, C//self.num_heads).permute(0,2,1,3)key = self.keyembedding(x).view(B,-1,self.key_dim)kv = self.kv(key).reshape(B,N,2,self.num_heads,C//self.num_heads).permute(2,0,3,1,4)k = kv[0]v = kv[1]q = q * self.scaleattn = (q @ k.transpose(-2, -1))relative_position_bias = self.relative_position_bias_table[self.relative_position_index.view(-1)].view(self.seq_len * self.seq_len, self.seq_len * self.seq_len, -1)  # Wh*Ww,Wh*Ww,nHrelative_position_bias = relative_position_bias.permute(2, 0, 1).contiguous()  # nH, Wh*Ww, Wh*Wwattn = attn + relative_position_bias.unsqueeze(0)attn = self.softmax(attn)attn = self.attn_drop(attn)x = (attn @ v).transpose(1, 2).reshape(B, N, C) x = self.proj(x)x = self.proj_drop(x)return xdef extra_repr(self) -> str:return f'dim={self.dim}, window_size={self.window_size}, num_heads={self.num_heads}'def flops(self, N):# calculate flops for 1 window with token length of Nflops = 0# qkv = self.qkv(x)flops += N * self.dim * 3 * self.dim# attn = (q @ k.transpose(-2, -1))flops += self.num_heads * N * (self.dim // self.num_heads) * N#  x = (attn @ v)flops += self.num_heads * N * N * (self.dim // self.num_heads)# x = self.proj(x)flops += N * self.dim * self.dimreturn flopsclass LocalTransformerBlock(nn.Module):r""" Local Transformer Block.Args:dim (int): Number of input channels.input_resolution (tuple[int]): Input resulotion.num_heads (int): Number of attention heads.window_size (int): Window size.shift_size (int): Shift size for SW-MSA.mlp_ratio (float): Ratio of mlp hidden dim to embedding dim.qkv_bias (bool, optional): If True, add a learnable bias to query, key, value. Default: Trueqk_scale (float | None, optional): Override default qk scale of head_dim ** -0.5 if set.drop (float, optional): Dropout rate. Default: 0.0attn_drop (float, optional): Attention dropout rate. Default: 0.0drop_path (float, optional): Stochastic depth rate. Default: 0.0act_layer (nn.Module, optional): Activation layer. Default: nn.GELUnorm_layer (nn.Module, optional): Normalization layer.  Default: nn.LayerNorm"""def __init__(self, dim, input_resolution, num_heads, window_size=7,mlp_ratio=4., qkv_bias=True, qk_scale=None, drop=0., attn_drop=0., drop_path=0.,act_layer=nn.GELU, norm_layer=nn.LayerNorm):super().__init__()self.dim = dimself.input_resolution = input_resolutionself.num_heads = num_headsself.window_size = window_sizeself.mlp_ratio = mlp_ratioif min(self.input_resolution) <= self.window_size:# if window size is larger than input resolution, we don't partition windowsself.window_size = min(self.input_resolution)self.norm1 = norm_layer(dim)self.attn = WindowAttention(dim, window_size=to_2tuple(self.window_size), num_heads=num_heads,qkv_bias=qkv_bias, qk_scale=qk_scale, attn_drop=attn_drop, proj_drop=drop)self.drop_path = DropPath(drop_path) if drop_path > 0. else nn.Identity()self.norm2 = norm_layer(dim)mlp_hidden_dim = int(dim * mlp_ratio)self.mlp = Mlp(in_features=dim, hidden_features=mlp_hidden_dim, act_layer=act_layer, drop=drop)def forward(self, x):H, W = self.input_resolutionB, L, C = x.shapeassert L == H * W, "input feature has wrong size"shortcut = xx = self.norm1(x)x = x.view(B, H, W, C)x_windows = window_partition(x, self.window_size)  # nW*B, window_size, window_size, C x_windows = x_windows.view(-1, self.window_size * self.window_size, C)  # nW*B, window_size*window_size, C     attn_windows = self.attn(x_windows)  # nW*B, window_size*window_size, C    attn_windows = attn_windows.view(-1, self.window_size, self.window_size, C)x = window_reverse(attn_windows, self.window_size, H, W)  # B H' W' Cx = x.view(B, H * W, C)x = shortcut + self.drop_path(x)x = x + self.drop_path(self.mlp(self.norm2(x)))return xdef extra_repr(self) -> str:return f"dim={self.dim}, input_resolution={self.input_resolution}, num_heads={self.num_heads}, " \f"window_size={self.window_size}, mlp_ratio={self.mlp_ratio}"def flops(self):flops = 0H, W = self.input_resolution# norm1flops += self.dim * H * W# W-MSA/SW-MSAnW = H * W / self.window_size / self.window_sizeflops += nW * self.attn.flops(self.window_size * self.window_size)# mlpflops += 2 * H * W * self.dim * self.dim * self.mlp_ratio# norm2flops += self.dim * H * Wreturn flopsclass PatchMerging(nn.Module):""" Patch Merging Layer.Args:input_resolution (tuple[int]): Resolution of input feature.dim (int): Number of input channels.norm_layer (nn.Module, optional): Normalization layer.  Default: nn.LayerNorm"""def __init__(self, input_resolution, dim, norm_layer=nn.LayerNorm):super().__init__()self.input_resolution = input_resolutionself.dim = dimself.reduction = nn.Linear(4 * dim, 2 * dim, bias=False)self.norm = norm_layer(4 * dim)def forward(self, x):"""x: B, H*W, C"""H, W = self.input_resolutionB, L, C = x.shapeassert L == H * W, "input feature has wrong size"assert H % 2 == 0 and W % 2 == 0, f"x size ({H}*{W}) are not even."x = x.view(B, H, W, C)x0 = x[:, 0::2, 0::2, :]  # B H/2 W/2 Cx1 = x[:, 1::2, 0::2, :]  # B H/2 W/2 Cx2 = x[:, 0::2, 1::2, :]  # B H/2 W/2 Cx3 = x[:, 1::2, 1::2, :]  # B H/2 W/2 Cx = torch.cat([x0, x1, x2, x3], -1)  # B H/2 W/2 4*Cx = x.view(B, -1, 4 * C)  # B H/2*W/2 4*Cx = self.norm(x)x = self.reduction(x)return xdef extra_repr(self) -> str:return f"input_resolution={self.input_resolution}, dim={self.dim}"def flops(self):H, W = self.input_resolutionflops = H * W * self.dimflops += (H // 2) * (W // 2) * 4 * self.dim * 2 * self.dimreturn flopsclass BasicLayer(nn.Module):""" A basic Swin Transformer layer for one stage.Args:dim (int): Number of input channels.input_resolution (tuple[int]): Input resolution.depth (int): Number of blocks.num_heads (int): Number of attention heads.window_size (int): Local window size.mlp_ratio (float): Ratio of mlp hidden dim to embedding dim.qkv_bias (bool, optional): If True, add a learnable bias to query, key, value. Default: Trueqk_scale (float | None, optional): Override default qk scale of head_dim ** -0.5 if set.drop (float, optional): Dropout rate. Default: 0.0attn_drop (float, optional): Attention dropout rate. Default: 0.0drop_path (float | tuple[float], optional): Stochastic depth rate. Default: 0.0norm_layer (nn.Module, optional): Normalization layer. Default: nn.LayerNormdownsample (nn.Module | None, optional): Downsample layer at the end of the layer. Default: Noneuse_checkpoint (bool): Whether to use checkpointing to save memory. Default: False."""def __init__(self, dim, input_resolution, depth, num_heads, window_size,mlp_ratio=4., qkv_bias=True, qk_scale=None, drop=0., attn_drop=0.,drop_path=0., norm_layer=nn.LayerNorm, downsample=None, drop_path_global=0., use_checkpoint=False):super().__init__()self.dim = dimself.input_resolution = input_resolutionself.depth = depthself.use_checkpoint = use_checkpointself.window_size = window_sizeself.drop_path_gl = DropPath(drop_path_global) if drop_path_global > 0. else nn.Identity()# build blocksself.blocks = nn.ModuleList([LocalTransformerBlock(dim=dim, input_resolution=input_resolution,num_heads=num_heads, window_size=window_size,mlp_ratio=mlp_ratio,qkv_bias=qkv_bias, qk_scale=qk_scale,drop=drop, attn_drop=attn_drop,drop_path=drop_path[i] if isinstance(drop_path, list) else drop_path,norm_layer=norm_layer)for i in range(depth)])# patch merging layerif downsample is not None:if min(self.input_resolution) >= self.window_size:self.glb_attn = GlobalAttention(dim, to_2tuple(window_size), self.input_resolution, num_heads = num_heads, qkv_bias=qkv_bias, qk_scale=qk_scale, attn_drop=attn_drop, proj_drop=drop)self.post_conv = nn.Conv2d(dim, dim, 3, padding=1)self.norm1 = norm_layer(dim)self.norm2 = norm_layer(dim)else:self.post_conv = Noneself.glb_attn = Noneself.norm1 = Noneself.norm2 = Noneself.downsample = downsample(input_resolution, dim=dim, norm_layer=norm_layer)else:self.downsample = Nonedef forward(self, x):for blk in self.blocks:if self.use_checkpoint:x = checkpoint.checkpoint(blk, x)else:x = blk(x)if self.downsample is not None:if min(self.input_resolution) >= self.window_size:shortcut = xx = self.norm1(x)H, W = self.input_resolutionB,_,C = x.size()no_window = int(H*W/self.window_size**2)   local_attn = x.view(B,no_window,self.window_size, self.window_size,C)glb_attn = self.glb_attn(x, H, W)glb_attn = glb_attn.view(B,no_window,1,1,C)x = torch.add(local_attn, glb_attn).view(B,C,H,W)x = shortcut.view(B,C,H,W) + self.drop_path_gl(x)x = self.norm2(x.view(B,H*W,C))post_conv = self.drop_path_gl(self.post_conv(x.view(B,C,H,W))).view(B, H*W, C)x = x + post_convx = self.downsample(x)return xdef extra_repr(self) -> str:return f"dim={self.dim}, input_resolution={self.input_resolution}, depth={self.depth}"def flops(self):flops = 0for blk in self.blocks:flops += blk.flops()if self.downsample is not None:flops += self.downsample.flops()return flopsclass PatchEmbed(nn.Module):r""" Image to Patch EmbeddingArgs:img_size (int): Image size.  Default: 224.patch_size (int): Patch token size. Default: 4.in_chans (int): Number of input image channels. Default: 3.embed_dim (int): Number of linear projection output channels. Default: 96.norm_layer (nn.Module, optional): Normalization layer. Default: None"""def __init__(self, img_size=224, patch_size=4, in_chans=3, embed_dim=96, norm_layer=None):super().__init__()img_size = to_2tuple(img_size)patch_size = to_2tuple(patch_size)patches_resolution = [img_size[0] // patch_size[0], img_size[1] // patch_size[1]]self.img_size = img_sizeself.patch_size = patch_sizeself.patches_resolution = patches_resolutionself.num_patches = patches_resolution[0] * patches_resolution[1]self.in_chans = in_chansself.embed_dim = embed_dimself.proj = nn.Conv2d(in_chans, embed_dim, kernel_size=patch_size, stride=patch_size)if norm_layer is not None:self.norm = norm_layer(embed_dim)else:self.norm = Nonedef forward(self, x):B, C, H, W = x.shape# FIXME look at relaxing size constraintsassert H == self.img_size[0] and W == self.img_size[1], \f"Input image size ({H}*{W}) doesn't match model ({self.img_size[0]}*{self.img_size[1]})."x = self.proj(x).flatten(2).transpose(1, 2)  # B Ph*Pw Cif self.norm is not None:x = self.norm(x)return xdef flops(self):Ho, Wo = self.patches_resolutionflops = Ho * Wo * self.embed_dim * self.in_chans * (self.patch_size[0] * self.patch_size[1])if self.norm is not None:flops += Ho * Wo * self.embed_dimreturn flopsclass MOATransformer(nn.Module):r""" Swin TransformerA PyTorch impl of : `Swin Transformer: Hierarchical Vision Transformer using Shifted Windows`  -https://arxiv.org/pdf/2103.14030Args:img_size (int | tuple(int)): Input image size. Default 224patch_size (int | tuple(int)): Patch size. Default: 4in_chans (int): Number of input image channels. Default: 3num_classes (int): Number of classes for classification head. Default: 1000embed_dim (int): Patch embedding dimension. Default: 96depths (tuple(int)): Depth of each Swin Transformer layer.num_heads (tuple(int)): Number of attention heads in different layers.window_size (int): Window size. Default: 7mlp_ratio (float): Ratio of mlp hidden dim to embedding dim. Default: 4qkv_bias (bool): If True, add a learnable bias to query, key, value. Default: Trueqk_scale (float): Override default qk scale of head_dim ** -0.5 if set. Default: Nonedrop_rate (float): Dropout rate. Default: 0attn_drop_rate (float): Attention dropout rate. Default: 0drop_path_rate (float): Stochastic depth rate. Default: 0.1norm_layer (nn.Module): Normalization layer. Default: nn.LayerNorm.ape (bool): If True, add absolute position embedding to the patch embedding. Default: Falsepatch_norm (bool): If True, add normalization after patch embedding. Default: Trueuse_checkpoint (bool): Whether to use checkpointing to save memory. Default: False"""def __init__(self, img_size=224, patch_size=4, in_chans=3, num_classes=1000,embed_dim=96, depths=[2, 2, 6, 2], num_heads=[3, 6, 12, 24],window_size=7, mlp_ratio=4., qkv_bias=True, qk_scale=None,drop_rate=0., attn_drop_rate=0., drop_path_rate=0.1,norm_layer=nn.LayerNorm, ape=False, patch_norm=True,use_checkpoint=False, **kwargs):super().__init__()self.num_classes = num_classesself.num_layers = len(depths)self.embed_dim = embed_dimself.ape = apeself.patch_norm = patch_normself.num_features = int(embed_dim * 2 ** (self.num_layers - 1))self.mlp_ratio = mlp_ratio# split image into non-overlapping patchesself.patch_embed = PatchEmbed(img_size=img_size, patch_size=patch_size, in_chans=in_chans, embed_dim=embed_dim,norm_layer=norm_layer if self.patch_norm else None)num_patches = self.patch_embed.num_patchespatches_resolution = self.patch_embed.patches_resolutionself.patches_resolution = patches_resolution# absolute position embeddingif self.ape:self.absolute_pos_embed = nn.Parameter(torch.zeros(1, num_patches, embed_dim))trunc_normal_(self.absolute_pos_embed, std=.02)self.pos_drop = nn.Dropout(p=drop_rate)# stochastic depthdpr = [x.item() for x in torch.linspace(0, drop_path_rate, sum(depths))]  # stochastic depth decay ruledpr_global = [x.item() for x in torch.linspace(0, 0.2, len(depths)-1)]# build layersself.layers = nn.ModuleList()for i_layer in range(self.num_layers):layer = BasicLayer(dim=int(embed_dim * 2 ** i_layer),input_resolution=(patches_resolution[0] // (2 ** i_layer),patches_resolution[1] // (2 ** i_layer)),depth=depths[i_layer],num_heads=num_heads[i_layer],window_size=window_size,mlp_ratio=self.mlp_ratio,qkv_bias=qkv_bias, qk_scale=qk_scale,drop=drop_rate, attn_drop=attn_drop_rate,drop_path=dpr[sum(depths[:i_layer]):sum(depths[:i_layer + 1])],norm_layer=norm_layer,downsample=PatchMerging if (i_layer < self.num_layers - 1) else None,drop_path_global = (dpr_global[i_layer]) if (i_layer < self.num_layers -1) else 0,use_checkpoint=use_checkpoint)self.layers.append(layer)self.norm = norm_layer(self.num_features)self.avgpool = nn.AdaptiveAvgPool1d(1)self.head = nn.Linear(self.num_features, num_classes) if num_classes > 0 else nn.Identity()self.apply(self._init_weights)def _init_weights(self, m):if isinstance(m, nn.Linear):trunc_normal_(m.weight, std=.02)if isinstance(m, nn.Linear) and m.bias is not None:nn.init.constant_(m.bias, 0)elif isinstance(m, nn.LayerNorm):nn.init.constant_(m.bias, 0)nn.init.constant_(m.weight, 1.0)@torch.jit.ignoredef no_weight_decay(self):return {'absolute_pos_embed'}@torch.jit.ignoredef no_weight_decay_keywords(self):return {'relative_position_bias_table'}def forward_features(self, x):x = self.patch_embed(x)if self.ape:x = x + self.absolute_pos_embedx = self.pos_drop(x)for layer in self.layers:x = layer(x)x = self.norm(x)  # B L Cx = self.avgpool(x.transpose(1, 2))  # B C 1x = torch.flatten(x, 1)return xdef forward(self, x):x = self.forward_features(x)x = self.head(x)return xdef flops(self):flops = 0flops += self.patch_embed.flops()for i, layer in enumerate(self.layers):flops += layer.flops()flops += self.num_features * self.patches_resolution[0] * self.patches_resolution[1] // (2 ** self.num_layers)flops += self.num_features * self.num_classesreturn flops