pytorch中的MultiheadAttention类解析

悠悠 2022-11-12 10:58 275阅读 0赞

# 总结 #

*  文章来源：CSDN\_LawsonAbs
 *  暂无亮点，持续更新~
 *  LawsonAbs的认知与思考，望各位读者审慎阅读。

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forward()函数的输入参数：

> query(L,N,E) L 指的是输出目标序列的长度，N 就是batch\_size， E 就是每个单词的embedding 维度大小  
> key(S,N,E) S 指的是输入目标序列的长度（也就是max\_seq\_length），N 就是batch\_size， E 就是每个单词的embedding 维度大小  
> value(S,N,E) ，S 指的是输入目标序列的长度，N 是 batch\_size, E 是embedding的维度

输出参数：  
attn\_output(L,N,E)。

为什么源序列输入长度和目标序列输出长度不一致？  
这个是完全可能的。结合下张图来看：

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpdTE2NjU5_size_16_color_FFFFFF_t_70]  
输入长度取决于 Q 的列维度，输出长度取决于V的列维度，而两者完全没有关联。

# 源码分析 #

class MultiheadAttention(Module):
        r"""Allows the model to jointly attend to information from different representation subspaces. See reference: Attention Is All You Need Args: embed_dim: total dimension of the model. num_heads: parallel attention heads. dropout: a Dropout layer on attn_output_weights. Default: 0.0. bias: add bias as module parameter. Default: True. add_bias_kv: add bias to the key and value sequences at dim=0. add_zero_attn: add a new batch of zeros to the key and value sequences at dim=1. kdim: total number of features in key. Default: None. vdim: total number of features in value. Default: None. Note: if kdim and vdim are None, they will be set to embed_dim such that query, key, and value have the same number of features. Examples:: >>> multihead_attn = nn.MultiheadAttention(embed_dim, num_heads) >>> attn_output, attn_output_weights = multihead_attn(query, key, value) """
        bias_k: Optional[torch.Tensor]
        bias_v: Optional[torch.Tensor]
    
        def __init__(self, embed_dim, num_heads, dropout=0., bias=True, add_bias_kv=False, add_zero_attn=False, kdim=None, vdim=None):
            super(MultiheadAttention, self).__init__()
            # embed_dim 应该是输入的维度
            self.embed_dim = embed_dim
            # 如果没有单独为kdim,vdim设置维度，则设置成和 embed_dim 同样的值
            self.kdim = kdim if kdim is not None else embed_dim
            self.vdim = vdim if vdim is not None else embed_dim
    		# 判断最后的值是否相同，设置一个标签
            self._qkv_same_embed_dim = self.kdim == embed_dim and self.vdim == embed_dim
    		
            self.num_heads = num_heads
            self.dropout = dropout
            self.head_dim = embed_dim // num_heads
            assert self.head_dim * num_heads == self.embed_dim, "embed_dim must be divisible by num_heads"
    
            if self._qkv_same_embed_dim is False:
            	# 这里默认Wq 矩阵的第二维度是 embed_dim
                self.q_proj_weight = Parameter(torch.Tensor(embed_dim, embed_dim))
                self.k_proj_weight = Parameter(torch.Tensor(embed_dim, self.kdim))
                self.v_proj_weight = Parameter(torch.Tensor(embed_dim, self.vdim))
                self.register_parameter('in_proj_weight', None)
            else: # 这么做是为了方便一次性初始化
                self.in_proj_weight = Parameter(torch.empty(3 * embed_dim, embed_dim))
                self.register_parameter('q_proj_weight', None)
                self.register_parameter('k_proj_weight', None)
                self.register_parameter('v_proj_weight', None)
    
            if bias:
                self.in_proj_bias = Parameter(torch.empty(3 * embed_dim))
            else:
                self.register_parameter('in_proj_bias', None)
                
            self.out_proj = _LinearWithBias(embed_dim, embed_dim)
    
            if add_bias_kv:
                self.bias_k = Parameter(torch.empty(1, 1, embed_dim))
                self.bias_v = Parameter(torch.empty(1, 1, embed_dim))
            else:
                self.bias_k = self.bias_v = None
    
            self.add_zero_attn = add_zero_attn
    
            self._reset_parameters()
    	
    	# 这是pytorch 中专用的初始化参数的方法
        def _reset_parameters(self):
            if self._qkv_same_embed_dim:
                xavier_uniform_(self.in_proj_weight)
            else:
                xavier_uniform_(self.q_proj_weight)
                xavier_uniform_(self.k_proj_weight)
                xavier_uniform_(self.v_proj_weight)
    
            if self.in_proj_bias is not None:
                constant_(self.in_proj_bias, 0.)
                constant_(self.out_proj.bias, 0.)
            if self.bias_k is not None:
                xavier_normal_(self.bias_k)
            if self.bias_v is not None:
                xavier_normal_(self.bias_v)
    
        def __setstate__(self, state):
            # Support loading old MultiheadAttention checkpoints generated by v1.1.0
            if '_qkv_same_embed_dim' not in state:
                state['_qkv_same_embed_dim'] = True
    
            super(MultiheadAttention, self).__setstate__(state)
    
        def forward(self, query, key, value, key_padding_mask=None,
                    need_weights=True, attn_mask=None):
            # type: (Tensor, Tensor, Tensor, Optional[Tensor], bool, Optional[Tensor]) -> Tuple[Tensor, Optional[Tensor]]
            r""" Args: query, key, value: map a query and a set of key-value pairs to an output. See "Attention Is All You Need" for more details. key_padding_mask: if provided, specified padding elements in the key will be ignored by the attention. When given a binary mask and a value is True, the corresponding value on the attention layer will be ignored. When given a byte mask and a value is non-zero, the corresponding value on the attention layer will be ignored need_weights: output attn_output_weights. attn_mask: 2D or 3D mask that prevents attention to certain positions. A 2D mask will be broadcasted for all the batches while a 3D mask allows to specify a different mask for the entries of each batch. Shape: - Inputs: - query: :math:`(L, N, E)` where L is the target sequence length, N is the batch size, E is the embedding dimension. - key: :math:`(S, N, E)`, where S is the source sequence length, N is the batch size, E is the embedding dimension. - value: :math:`(S, N, E)` where S is the source sequence length, N is the batch size, E is the embedding dimension. - key_padding_mask: :math:`(N, S)` where N is the batch size, S is the source sequence length. If a ByteTensor is provided, the non-zero positions will be ignored while the position with the zero positions will be unchanged. If a BoolTensor is provided, the positions with the value of ``True`` will be ignored while the position with the value of ``False`` will be unchanged. - attn_mask: 2D mask :math:`(L, S)` where L is the target sequence length, S is the source sequence length. 3D mask :math:`(N*num_heads, L, S)` where N is the batch size, L is the target sequence length, S is the source sequence length. attn_mask ensure that position i is allowed to attend the unmasked positions. If a ByteTensor is provided, the non-zero positions are not allowed to attend while the zero positions will be unchanged. If a BoolTensor is provided, positions with ``True`` is not allowed to attend while ``False`` values will be unchanged. If a FloatTensor is provided, it will be added to the attention weight. - Outputs: - attn_output: :math:`(L, N, E)` where L is the target sequence length, N is the batch size, E is the embedding dimension. - attn_output_weights: :math:`(N, L, S)` where N is the batch size, L is the target sequence length, S is the source sequence length. """
            if not self._qkv_same_embed_dim:
                return F.multi_head_attention_forward(
                    query, key, value, self.embed_dim, self.num_heads,
                    self.in_proj_weight, self.in_proj_bias,
                    self.bias_k, self.bias_v, self.add_zero_attn,
                    self.dropout, self.out_proj.weight, self.out_proj.bias,
                    training=self.training,
                    key_padding_mask=key_padding_mask, need_weights=need_weights,
                    attn_mask=attn_mask, use_separate_proj_weight=True,
                    q_proj_weight=self.q_proj_weight, k_proj_weight=self.k_proj_weight,
                    v_proj_weight=self.v_proj_weight)
            else:
                return F.multi_head_attention_forward(
                    query, key, value, self.embed_dim, self.num_heads,
                    self.in_proj_weight, self.in_proj_bias,
                    self.bias_k, self.bias_v, self.add_zero_attn,
                    self.dropout, self.out_proj.weight, self.out_proj.bias,
                    training=self.training,
                    key_padding_mask=key_padding_mask, need_weights=need_weights,
                    attn_mask=attn_mask)

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpdTE2NjU5_size_16_color_FFFFFF_t_70]: /images/20221022/f9db93c9322245999ebd2ace786bc2b6.png