使用pytorch实现transformer模型实战案例

好的，下面提供一个使用PyTorch实现Transformer模型的实战案例。

首先，我们需要导入必要的库和模块：

import math
import torch
import torch.nn as nn
import torch.nn.functional as F
from torch.autograd import Variable

接着，我们定义一些超参数：

d_model = 512  # 模型维度
n_layers = 6  # 层数
n_heads = 8  # 头数
dropout = 0.1  # Dropout概率

定义位置编码函数：

class PositionalEncoding(nn.Module):
    def __init__(self, d_model, dropout=0.1, max_len=5000):
        super(PositionalEncoding, self).__init__()
        self.dropout = nn.Dropout(p=dropout)

        pe = torch.zeros(max_len, d_model)
        position = torch.arange(0, max_len).unsqueeze(1).float()
        div_term = torch.exp(torch.arange(0, d_model, 2).float() * -(math.log(10000.0) / d_model))

        pe[:, 0::2] = torch.sin(position * div_term)
        pe[:, 1::2] = torch.cos(position * div_term)
        pe = pe.unsqueeze(0)

        self.register_buffer('pe', pe)

    def forward(self, x):
        x = x + Variable(self.pe[:, :x.size(1)], requires_grad=False)
        return self.dropout(x)

定义多头自注意力机制模块：

class MultiHeadedAttention(nn.Module):
    def __init__(self, n_heads, d_model, dropout=0.1):
        super(MultiHeadedAttention, self).__init__()
        assert d_model % n_heads == 0
        self.d_k = d_model // n_heads
        self.n_heads = n_heads

        self.linears = nn.ModuleList([nn.Linear(d_model, d_model) for _ in range(4)])
        self.dropout = nn.Dropout(p=dropout)

    def attention(self, q, k, v, mask=None, dropout=None):
        scores = torch.matmul(q, k.transpose(-2, -1)) / math.sqrt(self.d_k)
        if mask is not None:
            scores = scores.masked_fill(mask == 0, -1e9)
        p_attn = F.softmax(scores, dim=-1)
        if dropout is not None:
            p_attn = dropout(p_attn)
        return torch.matmul(p_attn, v), p_attn

    def forward(self, query, key, value, mask=None):
        if mask is not None:
            mask = mask.unsqueeze(1)
        nbatches = query.size(0)

        # 1) Do all the linear projections in batch from d_model => h x d_k
        query, key, value = [l(x).view(nbatches, -1, self.n_heads, self.d_k).transpose(1, 2)
                             for l, x in zip(self.linears, (query, key, value))]

        # 2) Apply attention on all the projected vectors in batch.
        x, attn = self.attention(query, key, value, mask=mask, dropout=self.dropout)

        # 3) "Concat" using a view and apply a final linear.
        x = x.transpose(1, 2).contiguous().view(nbatches, -1, self.n_heads * self.d_k)
        return self.linears[-1](x), attn

定义前馈神经网络模块：

class PositionwiseFeedForward(nn.Module):
    def __init__(self, d_model, d_ff, dropout=0.1):
        super(PositionwiseFeedForward, self).__init__()
        self.w_1 = nn.Linear(d_model, d_ff)
        self.w_2 = nn.Linear(d_ff, d_model)
        self.dropout = nn.Dropout(dropout)

    def forward(self, x):
        return self.w_2(self.dropout(F.relu(self.w_1(x))))

定义一个Transformer层模块：

class TransformerLayer(nn.Module):
    def __init__(self, d_model, n_heads, dropout=0.1):
        super(TransformerLayer, self).__init__()
        self.self_attn = MultiHeadedAttention(n_heads, d_model, dropout=dropout)
        self.feed_forward = PositionwiseFeedForward(d_model, d_model*4, dropout=dropout)
        self.sublayer = nn.ModuleList([SublayerConnection(d_model, dropout=dropout) for _ in range(2)])
        self.d_model = d_model

    def forward(self, x, mask):
        x, _ = self.self_attn(x, x, x, mask=mask)
        x = self.sublayer[0](x, lambda x: self.self_attn(x, x, x, mask=mask))
        x = self.sublayer[1](x, self.feed_forward)
        return x

定义一个包含若干个Transformer层的Transformer模块：

class Transformer(nn.Module):
    def __init__(self, n_layers, d_model, n_heads, dropout=0.1):
        super(Transformer, self).__init__()
        self.layers = nn.ModuleList([TransformerLayer(d_model, n_heads, dropout=dropout) for _ in range(n_layers)])
        self.norm = nn.LayerNorm(d_model)

    def forward(self, x, mask):
        for layer in self.layers:
            x = layer(x, mask)
        return self.norm(x)

定义一个SublayerConnection模块，用于连接子层：

class SublayerConnection(nn.Module):
    def __init__(self, size, dropout=0.1):
        super(SublayerConnection, self).__init__()
        self.norm = nn.LayerNorm(size)
        self.dropout = nn.Dropout(dropout)

    def forward(self, x, sublayer):
        return x + self.dropout(sublayer(self.norm(x)))

最后，我们可以使用这些模块来构建一个Transformer模型：

class TransformerModel(nn.Module):
    def __init__(self, vocab_size, d_model, n_layers, n_heads, dropout=0.1):
        super(TransformerModel, self).__init__()
        self.embedding = nn.Embedding(vocab_size, d_model)
        self.pos_encoder = PositionalEncoding(d_model, dropout=dropout)
        self.transformer = Transformer(n_layers, d_model, n_heads, dropout=dropout)
        self.decoder = nn.Linear(d_model, vocab_size)

        self.init_weights()

    def init_weights(self):
        initrange = 0.1
        self.embedding.weight.data.uniform_(-initrange, initrange)
        self.decoder.bias.data.zero_()
        self.decoder.weight.data.uniform_(-initrange, initrange)

    def forward(self, src, mask):
        src = self.embedding(src)
        src = self.pos_encoder(src)
        output = self.transformer(src, mask)
        output = self.decoder(output)
        return output

以上就是使用PyTorch实现Transformer模型的实战案例。可以根据具体的需求进行修改和扩展。