cntk Embedding layer
“Embedding” refers to representing words or other discrete items by dense continuous vectors. This layer assumes that the input is in one-hot form. E.g., for a vocabulary size of 10,000, each input vector is expected to have dimension 10,000 and consist of zeroes except for one position that contains a 1. The index of that location is the index of the word or item it represents.
通过上面一段话，Embedding layer的工作就是把 one_hot类型的向量转换为dense continuous vector。该转换过程是通过矩阵相乘实现的，但是当 vocabulary size 很大时， 为了提高矩阵乘法的效率，在进行相乘之前，需要将one_hot类型向量变换为稀疏形式的写法，这种转换是通过参数 is_sparse=True来实现的：input = C.input_variable(shape=(784, ), is_sparse=True)
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