构建一个完整的BERT模型并进行训练是一个复杂且耗时的任务。BERT模型由多个组件组成，包括嵌入层、Transformer编码器和分类器等。编写这些组件的完整代码超出了文本的范围。然而，一个基本的BERT模型框架以便了解其结构和主要组件的设置。

import torch
import torch.nn as nn# BERT Model
class BERTModel(nn.Module):def __init__(self, vocab_size, embedding_dim, hidden_dim, num_layers, num_heads, max_seq_length, num_classes):super(BERTModel, self).__init__()self.embedding = nn.Embedding(vocab_size, embedding_dim)self.position_embedding = nn.Embedding(max_seq_length, embedding_dim)self.transformer_blocks = nn.ModuleList([TransformerBlock(embedding_dim, hidden_dim, num_heads)for _ in range(num_layers)])self.classifier = nn.Linear(embedding_dim, num_classes)self.dropout = nn.Dropout(p=0.1)def forward(self, input_ids, attention_mask):embedded = self.embedding(input_ids)  # [batch_size, seq_length, embedding_dim]positions = torch.arange(0, input_ids.size(1), device=input_ids.device).unsqueeze(0).expand_as(input_ids)position_embedded = self.position_embedding(positions)  # [batch_size, seq_length, embedding_dim]encoded = self.dropout(embedded + position_embedded)  # [batch_size, seq_length, embedding_dim]for transformer_block in self.transformer_blocks:encoded = transformer_block(encoded, attention_mask)pooled_output = encoded[:, 0, :]  # [batch_size, embedding_dim]logits = self.classifier(pooled_output)  # [batch_size, num_classes]return logits# Transformer Block
class TransformerBlock(nn.Module):def __init__(self, embedding_dim, hidden_dim, num_heads):super(TransformerBlock, self).__init__()self.attention = MultiHeadAttention(embedding_dim, num_heads)self.feed_forward = FeedForward(hidden_dim, embedding_dim)self.layer_norm1 = nn.LayerNorm(embedding_dim)self.layer_norm2 = nn.LayerNorm(embedding_dim)def forward(self, x, attention_mask):attended = self.attention(x, x, x, attention_mask)  # [batch_size, seq_length, embedding_dim]residual1 = x + attendednormalized1 = self.layer_norm1(residual1)  # [batch_size, seq_length, embedding_dim]fed_forward = self.feed_forward(normalized1)  # [batch_size, seq_length, embedding_dim]residual2 = normalized1 + fed_forwardnormalized2 = self.layer_norm2(residual2)  # [batch_size, seq_length, embedding_dim]return normalized2# Multi-Head Attention
class MultiHeadAttention(nn.Module):def __init__(self, embedding_dim, num_heads):super(MultiHeadAttention, self).__init__()self.num_heads = num_headsself.head_dim = embedding_dim // num_headsself.q_linear = nn.Linear(embedding_dim, embedding_dim)self.k_linear = nn.Linear(embedding_dim, embedding_dim)self.v_linear = nn.Linear(embedding_dim, embedding_dim)self.out_linear = nn.Linear(embedding_dim, embedding_dim)def forward(self, query, key, value, mask=None):batch_size = query.size(0)query = self.q_linear(query)  # [batch_size, seq_length, embedding_dim]key = self.k_linear(key)  # [batch_size, seq_length, embedding_dim]value = self.v_linear(value)  # [batch_size, seq_length, embedding_dim]query = self._split_heads(query)  # [batch_size, num_heads, seq_length, head_dim]key = self._split_heads(key)  # [batch_size, num_heads, seq_length, head_dim]value = self._split_heads(value)  # [batch_size, num_heads, seq_length, head_dim]scores = torch.matmul(query, key.transpose(-1, -2))  # [batch_size, num_heads, seq_length, seq_length]scores = scores / torch.sqrt(torch.tensor(self.head_dim, dtype=torch.float32, device=scores.device))if mask is not None:scores = scores.masked_fill(mask.unsqueeze(1).unsqueeze(2), -1e9)attention_outputs = torch.softmax(scores, dim=-1)  # [batch_size, num_heads, seq_length, seq_length]attention_outputs = self.dropout(attention_outputs)attended = torch.matmul(attention_outputs, value)  # [batch_size, num_heads, seq_length, head_dim]attended = attended.transpose(1, 2).contiguous()  # [batch_size, seq_length, num_heads, head_dim]attended = attended.view(batch_size, -1, self.embedding_dim)  # [batch_size, seq_length, embedding_dim]attended = self.out_linear(attended)  # [batch_size, seq_length, embedding_dim]return attendeddef _split_heads(self, x):batch_size, seq_length, embedding_dim = x.size()x = x.view(batch_size, seq_length, self.num_heads, self.head_dim)x = x.transpose(1, 2).contiguous()return x# Feed Forward
class FeedForward(nn.Module):def __init__(self, hidden_dim, embedding_dim):super(FeedForward, self).__init__()self.linear1 = nn.Linear(embedding_dim, hidden_dim)self.activation = nn.ReLU()self.dropout = nn.Dropout(p=0.1)self.linear2 = nn.Linear(hidden_dim, embedding_dim)def forward(self, x):x = self.linear1(x)  # [batch_size, seq_length, hidden_dim]x = self.activation(x)x = self.dropout(x)x = self.linear2(x)  # [batch_size, seq_length, embedding_dim]return x# Example usage
vocab_size = 10000
embedding_dim = 300
hidden_dim = 768
num_layers = 12
num_heads = 12
max_seq_length = 512
num_classes = 2model = BERTModel(vocab_size, embedding_dim, hidden_dim, num_layers, num_heads, max_seq_length, num_classes)
input_ids = torch.tensor([[1, 2, 3, 4, 5]]).long()
attention_mask = torch.tensor([[1, 1, 1, 1, 1]]).long()
logits = model(input_ids, attention_mask)
print(logits.shape)  # [1, num_classes]

这段代码给出了一个基本的BERT模型结构，并包含了Transformer块、注意力机制和前馈神经网络等组件。您需要根据自己的需求和数据集来调整参数和模型结构。

请注意，这只是一个简化的版本，真实的BERT模型还包括Masked Language Modeling（MLM）和Next Sentence Prediction（NSP）等预训练任务。此外，还需要进行数据预处理、损失函数的定义和训练循环等。在实际环境中，强烈建议使用已经经过大规模预训练的BERT模型，如Hugging Face的transformers库中的预训练模型，以获得更好的性能效果。