语义匹配，cnn + self-attention孪生网络

一.模型结构

实现一个简单的孪生网络（结构如上图），做语义相似度：

    1.从上图可看出整体的结构相对较简单，左右两边基本一致。A句和B句分别进入左右两个结构。输入到网络中是token embedding + position_embedding

    2.再经过cnn-encoder进行编码

    3.多头注意力层，self-attention的输入：一个是本句cnn-encoder的输出；一个是另一句的cnn-encoder的输出。作为两句的交互层

    4.将cnn-encoder的输出和self-attention的输出进行cat连接

    5.接一个fc层

    6.一个平均池化层

    7.最后是用cosine余弦作相似度匹配计算

cnn-encoder结构如下：

二.程序：（完整项目见：https://github.com/jiangnanboy/semantic_matching/tree/master/model1）

# 搭建模型

class Encoder(nn.Module):
    def __init__(self, input_dim, hid_dim, n_layers, kernel_size, dropout, max_length=30):
        super(Encoder, self).__init__()
        
        #for kernel in kernel_size:
        assert kernel_size % 2 == 1,'kernel size must be odd!' # 卷积核size为奇数，方便序列两边pad处理
        
        self.scale = torch.sqrt(torch.FloatTensor([0.5])).to(DEVICE) # 确保整个网络的方差不会发生显著变化
        
        self.tok_embedding = nn.Embedding(input_dim, hid_dim) # token编码
        self.pos_embedding = nn.Embedding(max_length, hid_dim) # token的位置编码
        
        #self.emb2hid = nn.Linear(emb_dim, hid_dim) # 线性层，从emb_dim转为hid_dim
        #self.hid2emb = nn.Linear(hid_dim, emb_dim) # 线性层，从hid_dim转为emb_dim
        
        # 卷积块
        
        self.convs = nn.ModuleList([nn.Conv1d(in_channels=hid_dim,
                                              out_channels=2*hid_dim, # 卷积后输出的维度，这里2*hid_dim是为了后面的glu激活函数
                                              kernel_size=kernel_size,
                                              padding=(kernel_size - 1)//2) # 序列两边补0个数，保持维度不变
                                              for _ in range(n_layers)]) 
        
        '''
        利用不同size的卷积核进行特征提取
        self.conv_1 = nn.ModuleList([nn.Conv1d(in_channels=hid_dim,
                                                  out_channels=2*hid_dim, # 卷积后输出的维度，这里2*hid_dim是为了后面的glu激活函数
                                                  kernel_size=kernel_size[0],
                                                  padding=(kernel_size[0] - 1)//2) # 序列两边补0个数，保持维度不变
                                                  for _ in range(n_layers)])
        self.conv_2 = nn.ModuleList([nn.Conv1d(in_channels=hid_dim,
                                                  out_channels=2*hid_dim, # 卷积后输出的维度，这里2*hid_dim是为了后面的glu激活函数
                                                  kernel_size=kernel_size[1],
                                                  padding=(kernel_size[1] - 1)//2) # 序列两边补0个数，保持维度不变
                                                  for _ in range(n_layers)])
        self.conv_3 = nn.ModuleList([nn.Conv1d(in_channels=hid_dim,
                                                  out_channels=2*hid_dim, # 卷积后输出的维度，这里2*hid_dim是为了后面的glu激活函数
                                                  kernel_size=kernel_size[2],
                                                  padding=(kernel_size[2] - 1)//2) # 序列两边补0个数，保持维度不变
                                                  for _ in range(n_layers)])
        
        # 几个卷积模块转换维度
        self.convhid2hid = nn.Linear(len(kernel_size) * hid_dim, hid_dim)
        '''
        
        self.dropout = nn.Dropout(dropout)
        
    def forward(self, src):
        # src: [batch_size, src_len]
        batch_size = src.shape[0]
        src_len = src.shape[1]
        
        # 创建token位置信息
        pos = torch.arange(src_len).unsqueeze(0).repeat(batch_size, 1).to(DEVICE) # [batch_size, src_len]
        
        # 对token与其位置进行编码
        tok_embedded = self.tok_embedding(src) # [batch_size, src_len, emb_dim]
        pos_embedded = self.pos_embedding(pos.long()) # [batch_size, src_len, emb_dim]
        
        # 对token embedded和pos_embedded逐元素加和
        embedded = self.dropout(tok_embedded + pos_embedded) # [batch_size, src_len, emb_dim]
        
        # embedded经过一线性层，将emb_dim转为hid_dim，作为卷积块的输入
        #conv_input = self.emb2hid(embedded) # [batch_size, src_len, hid_dim]
        
        # 转变维度，卷积在输入数据的最后一维进行
        conv_input = embedded.permute(0, 2, 1) # [batch_size, hid_dim, src_len]
        
        
        # 以下进行卷积块
        for i, conv in enumerate(self.convs):
            # 进行卷积
            conved = conv(self.dropout(conv_input)) # [batch_size, 2*hid_dim, src_len]
            
            # 进行激活glu
            conved = F.glu(conved, dim=1) # [batch_size, hid_dim, src_len]
            
            # 进行残差连接
            conved = (conved + conv_input) * self.scale # [batch_size, hid_dim, src_len]
            
            # 作为下一个卷积块的输入
            conv_input = conved
        
        # 经过一线性层，将hid_dim转为emb_dim，作为enocder的卷积输出的特征
        #conved = self.hid2emb(conved.permute(0, 2, 1)) # [batch_size, src_len, emb_dim]
        
        
        
        '''
        利用不同size的卷积核进行特征提取
        # 第一个kernel_size
        conved_input = conv_input
        for i, conv in enumerate(self.conv_1):
            # 进行卷积
            conved1 = conv(self.dropout(conved_input)) # [batch_size, 2*hid_dim, src_len]

            # 进行激活glu
            conved1 = F.glu(conved1, dim=1) # [batch_size, hid_dim, src_len]

            # 进行残差连接
            conved1 = (conved1 + conved_input) * self.scale # [batch_size, hid_dim, src_len]

            # 作为下一个卷积块的输入
            conved_input = conved1
        
        combine_conv_module = conved1
        
        # 第二个kernel_size
        conved_input = conv_input
        for i, conv in enumerate(self.conv_2):
            # 进行卷积
            conved2 = conv(self.dropout(conved_input)) # [batch_size, 2*hid_dim, src_len]

            # 进行激活glu
            conved2 = F.glu(conved2, dim=1) # [batch_size, hid_dim, src_len]

            # 进行残差连接
            conved2 = (conved2 + conved_input) * self.scale # [batch_size, hid_dim, src_len]

            # 作为下一个卷积块的输入
            conved_input = conved2
            
        combine_conv_module = torch.cat([combine_conv_module, conved2], dim = 1)
        
        # 第三个kernel_size
        conved_input = conv_input
        for i, conv in enumerate(self.conv_3):
            # 进行卷积
            conved3 = conv(self.dropout(conved_input)) # [batch_size, 2*hid_dim, src_len]

            # 进行激活glu
            conved3 = F.glu(conved3, dim=1) # [batch_size, hid_dim, src_len]

            # 进行残差连接
            conved3 = (conved3 + conved_input) * self.scale # [batch_size, hid_dim, src_len]

            # 作为下一个卷积块的输入
            conved_input = conved3
            
        combine_conv_module = torch.cat([combine_conv_module, conved3], dim = 1)
        
        conved = self.convhid2hid(combine_conv_module.permute(0, 2, 1)) # [batch_size, src_len, hid_dim]
        '''
        
        # 又是一个残差连接，逐元素加和输出，作为encoder的联合输出特征
        combined = (conved.permute(0, 2, 1) + embedded) * self.scale # [batch_size, src_len, emb_dim]
        
        return conved, combined

'''
多头注意力multi-head attention
'''
class MultiHeadAttentionLayer(nn.Module):
    def __init__(self, hid_dim, n_heads, dropout):
        super(MultiHeadAttentionLayer, self).__init__()
        
        assert hid_dim % n_heads == 0
        
        self.hid_dim = hid_dim
        self.n_heads = n_heads
        self.head_dim = hid_dim // n_heads
        
        self.fc_q = nn.Linear(hid_dim, hid_dim)
        self.fc_k = nn.Linear(hid_dim, hid_dim)
        self.fc_v = nn.Linear(hid_dim, hid_dim)
        
        self.fc_o = nn.Linear(hid_dim, hid_dim)
        
        self.dropout = nn.Dropout(dropout)
        
        self.scale = torch.sqrt(torch.FloatTensor([self.hid_dim])).to(DEVICE) # 缩放因子
        
    def forward(self, query, key, value, mask=None):
        '''
        query: [batch_size, query_len, hid_dim]
        key: [batch_size, key_len, hid_dim]
        value: [batch_size, value_len, hid_dim]
        '''
        batch_size = query.shape[0]
        
        Q = self.fc_q(query) # [batch_size, query_len, hid_dim]
        K = self.fc_k(key) # [batch_size, key_len, hid_dim]
        V = self.fc_v(value) # [batch_size, value_len, hid_dim]
        
        Q = Q.view(batch_size, -1, self.n_heads, self.head_dim).permute(0, 2, 1, 3) # [batch_size, n_heads, query_len, head_dim]
        K = K.view(batch_size, -1, self.n_heads, self.head_dim).permute(0, 2, 1, 3) # [batch_size, n_heads, key_len, head_dim]
        V = V.view(batch_size, -1, self.n_heads, self.head_dim).permute(0, 2, 1, 3) # [batch_size, n_heads, value_len, head_dim]
        
        # [batch_size, n_heads, query_len, head_dim] * [batch_size, n_heads, head_dim, key_len]
        energy = torch.matmul(Q, K.permute(0, 1, 3, 2)) / self.scale # [batch_size, n_heads, query_len, key_len]
        
        if mask != None:
            energy = energy.masked_fill(mask == 0, -1e10)
        
        attention = torch.softmax(energy, dim=-1) # [batch_size, n_heads, query_len, key_len]
        
        # [batch_size, n_heads, query_len, key_len] * [batch_size, n_heads, value_len, head_dim]
        x = torch.matmul(self.dropout(attention), V) # [batch_size, n_heads, query_len, head_dim]
        
        x = x.permute(0, 2, 1, 3).contiguous() # [batch_size, query_len, n_heads, head_dim]
        
        x = x.view(batch_size, -1, self.hid_dim) # [batch_size, query_len, hid_dim]
        
        x = self.fc_o(x) # [batch_size, query_len, hid_dim]
        
        return x, attention
    
class SiameseNetwork(nn.Module):
    def __init__(self, EncoderA, hid_dim, n_heads, dropout):
        super(SiameseNetwork, self).__init__()
        self.EncoderA = EncoderA
        #self.EncoderB = EncoderB
        #self.dropout = nn.Dropout(dropout)
        
        # 多头
        self.self_attention = MultiHeadAttentionLayer(hid_dim, n_heads, dropout)
        
        self.fcA = nn.Linear(2 * hid_dim, hid_dim)
        self.fcB = nn.Linear(2 * hid_dim, hid_dim)
        
        self.fc_out = nn.Linear(5 * hid_dim, 2)
        
    def calculate_attention(self, convedA, convedB):
        '''
        convedA:[batch_size, len, hid_dim]
        convedB:[batch_size, len, hid_dim]
        '''
        energy = torch.matmul(convedA, convedB.permute(0, 2, 1)) # [batch_size, trg_len, src_len]
        
        attention = F.softmax(energy, dim=2) # [batch_size, trg_len, src_len]
        
        attention_encoding = torch.matmul(attention, convedB) # [batch_size, trg_len, hid_dim]
        
        return attention, attention_encoding
    
    def forward(self, sentA, sentB):
        convedA, combinedA = self.EncoderA(sentA)
        convedB, combinedB = self.EncoderA(sentB)
        
        # 普通attention
        #attentionA, attended_encodingA = self.calculate_attention(combinedB, combinedA)
        #attentionB, attended_encodingB = self.calculate_attention(combinedA, combinedB)
        
        # 多头attention，来自transformer模型中
        self_attentionA, attentionA = self.self_attention(combinedB, combinedA, combinedA)
        self_attentionB, attentionB = self.self_attention(combinedA, combinedB, combinedB)
        
        combinedA = torch.cat([self_attentionA, combinedA], dim=2) # [batch_size, len, 2 * hid_dim]
        combinedB = torch.cat([self_attentionB, combinedB], dim=2) # [batch_size, len, 2 * hid_dim]
        
        combinedA = self.fcA(combinedA) # [batch_size, len, hid_dim]
        combinedB = self.fcB(combinedB) # [batch_size, len, hid_dim]
        
        combinedA = F.avg_pool1d(combinedA.permute(0, 2, 1), combinedA.shape[1]).squeeze(2) # [batch_size, emb_dim]
        combinedB = F.avg_pool1d(combinedB.permute(0, 2, 1), combinedB.shape[1]).squeeze(2) # [batch_size, emb_dim]

        similarity = torch.cosine_similarity(combinedA, combinedB, dim=1) # 直接计算和学习相似度
        
        # 以下是做二分类
        # [p, q, p+q, p-q, p*q]
        #fc_out = self.fc_out(torch.cat([combinedA, combinedB, combinedA+combinedB, combinedA-combinedB, combinedA*combinedB], dim=1)) # 【batch_size, 2】
        
        return similarity