NLP笔记（8）——轻松构建Seq2Seq模型，保姆级教学！

 一、Seq2Seq的原理

Sequence to sequence (seq2seq)是由encoder（编码器）和decoder（解码器）两个RNN的组成的。其中encoder负责对输入句子的理解，转化为context vector，decoder负责对理解后的句子的向量进行处理，解码，获得输出。上述的过程和我们大脑理解东西的过程很相似，听到一句话，理解之后，尝试组装答案，进行回答。那么此时，就有一个问题，在encoder的过程中得到的context vector作为decoder的输入，那么这样一个输入，怎么能够得到多个输出呢？其实就是当前一步的输出，作为下一个单元的输入，然后得到结果。

outputs = []
while True:
    output = decoderd(output)
    outputs.append(output)

在训练数据集中，可以再输出的最后面添加一个结束符<END>，如果遇到该结束符，则可以终止循环。

outputs = []
while output!="<END>":
    output = decoderd(output)
    outputs.append(output)

Seq2seq模型中的encoder接受一个长度为M的序列，得到1个 context vector，之后decoder把这一个context vector转化为长度为N的序列作为输出，从而构成一个M to N的模型，能够处理很多不定长输入输出的问题，比如：文本翻译，问答，文章摘要，关键字写诗等等

二、Seq2Seq模型的实现

2.1.模型需求及实现流程

需求：完成一个模型，实现往模型输入一串数字，输出这串数字+0

例如：

• 输入123456789，输出1234567890；

• 输入52555568，输出525555680；

流程：

首先文本转化为序列，使用序列，准备数据集，准备Dataloader。然后完成编码器和解码器。然后完成seq2seq模型。然后完成模型训练的逻辑，进行训练。然后完成模型评估的逻辑，进行模型评估。

2.2.模型的实现

1.创建配置文件（config.py）

batch_size = 512
max_len = 10
dropout = 0
embedding_dim = 100
hidden_size = 64

2.文本转化为序列（word_sequence.py）

由于输入的是数字，为了把这写数字和词典中的真实数字进行对应，可以把这些数字理解为字符串。所以需要先把字符串对应为数字，然后把数字转化为字符串。

class NumSequence:
    UNK_TAG = "UNK"
    PAD_TAG = "PAD"
    EOS_TAG = "EOS"
    SOS_TAG = "SOS"
    UNK = 0
    PAD = 1
    EOS = 2
    SOS = 3
    def __init__(self):
        self.dict = {
            self.UNK_TAG : self.UNK,
            self.PAD_TAG : self.PAD,
            self.EOS_TAG : self.EOS,
            self.SOS_TAG : self.SOS
        }
        for i in range(10):
            self.dict[str(i)] = len(self.dict)
        self.index2word = dict(zip(self.dict.values(),self.dict.keys()))
    def __len__(self):
        return len(self.dict)
    def transform(self,sequence,max_len=None,add_eos=False):
        sequence_list = list(str(sequence))
        seq_len = len(sequence_list)+1 if add_eos else len(sequence_list)
 
        if add_eos and max_len is not None:
            assert max_len>= seq_len, "max_len 需要大于seq+eos的长度"
        _sequence_index = [self.dict.get(i,self.UNK) for i in sequence_list]
        if add_eos:
            _sequence_index += [self.EOS]
        if max_len is not None:
            sequence_index = [self.PAD]*max_len
            sequence_index[:seq_len] =  _sequence_index
            return sequence_index
        else:
            return _sequence_index
    def inverse_transform(self,sequence_index):
        result = []
        for i in sequence_index:
            if i==self.EOS:
                break
            result.append(self.index2word.get(int(i),self.UNK_TAG))
        return result
# 实例化
num_sequence = NumSequence()
if __name__ == '__main__':
    num_sequence = NumSequence()
    print(num_sequence.dict)
    print(num_sequence.index2word)
    print(num_sequence.transform("1231230",add_eos=True))

3.数据集(dataset.py)

随机创建[0，100000000]的整型，准备数据集，运行程序可以看到大部分的数字长度为8，在目标值后面添加上0和EOS之后，最大长度为10。所以config配置文件的max_len=10。

from torch.utils.data import Dataset,DataLoader
import numpy as np
from word_sequence import num_sequence
import torch
import config
class RandomDataset(Dataset):
    def __init__(self):
        super(RandomDataset,self).__init__()
        self.total_data_size = 500000
        np.random.seed(10)
        self.total_data = np.random.randint(1,100000000,size=[self.total_data_size])
    def __getitem__(self, idx):
        input = str(self.total_data[idx])
        return input, input+ "0",len(input),len(input)+1
    def __len__(self):
        return self.total_data_size
def collate_fn(batch):
    #1. 对batch进行排序，按照长度从长到短的顺序排序
    batch = sorted(batch,key=lambda x:x[3],reverse=True)
    input,target,input_length,target_length = zip(*batch)
    #2.进行padding的操作
    input = torch.LongTensor([num_sequence.transform(i,max_len=config.max_len) for i in input])
    target = torch.LongTensor([num_sequence.transform(i,max_len=config.max_len,add_eos=True) for i in target])
    input_length = torch.LongTensor(input_length)
    target_length = torch.LongTensor(target_length)
    return input,target,input_length,target_length
data_loader = DataLoader(dataset=RandomDataset(),batch_size=config.batch_size,collate_fn=collate_fn,drop_last=True)
if __name__ == '__main__':
    data_loader = DataLoader(dataset=RandomDataset(),batch_size=config.batch_size,drop_last=True)
    for idx,(input,target,input_lenght,target_length) in enumerate(data_loader):
        print(idx) #输出
        print(input) #输入
        print(target) #输出，后面加0
        print(input_lenght) #输入长度
        print(target_length) #输出长度
        break

4.编码器(encoder.py)

编码器（encoder）的目的就是为了对文本进行编码，把编码后的结果交给后续的程序使用，所以在这里可以使用Embedding+GRU的结构，使用最后一个time step的输出(hidden state)作为句子的编码结果。

import torch.nn as nn
from word_sequence import num_sequence
import config
class NumEncoder(nn.Module):
    def __init__(self):
        super(NumEncoder,self).__init__()
        self.vocab_size = len(num_sequence)
        self.dropout = config.dropout
        self.embedding_dim = config.embedding_dim
        self.embedding = nn.Embedding(num_embeddings=self.vocab_size,embedding_dim=self.embedding_dim,padding_idx=num_sequence.PAD)
        self.gru = nn.GRU(input_size=self.embedding_dim,
                          hidden_size=config.hidden_size,
                          num_layers=1,
                          batch_first=True,
                          dropout=config.dropout)
    def forward(self, input,input_length):
        embeded = self.embedding(input)
        embeded = nn.utils.rnn.pack_padded_sequence(embeded,lengths=input_length,batch_first=True)
        out,hidden = self.gru(embeded)
        out,outputs_length = nn.utils.rnn.pad_packed_sequence(out,batch_first=True,padding_value=num_sequence.PAD)
        return out,hidden

5.解码器(decoder.py)

解码器主要负责实现对编码之后结果的处理，得到预测值，为后续计算损失做准备。解码器也是一个RNN，即也可以使用LSTM or GRU的结构。

import torch
import torch.nn as nn
import config
import random
import torch.nn.functional as F
from word_sequence import num_sequence
 
class NumDecoder(nn.Module):
    def __init__(self):
        super(NumDecoder,self).__init__()
        self.max_seq_len = config.max_len
        self.vocab_size = len(num_sequence)
        self.embedding_dim = config.embedding_dim
        self.dropout = config.dropout
 
        self.embedding = nn.Embedding(num_embeddings=self.vocab_size,embedding_dim=self.embedding_dim,padding_idx=num_sequence.PAD)
        self.gru = nn.GRU(input_size=self.embedding_dim,
                          hidden_size=config.hidden_size,
                          num_layers=1,
                          batch_first=True,
                          dropout=self.dropout)
        self.log_softmax = nn.LogSoftmax()
 
        self.fc = nn.Linear(config.hidden_size,self.vocab_size)
 
    def forward(self, encoder_hidden,target,target_length):
        # encoder_hidden [batch_size,hidden_size]
        # target [batch_size,seq-len]
 
        decoder_input = torch.LongTensor([[num_sequence.SOS]]*config.batch_size)
        # print("decoder_input size:",decoder_input.size())
        decoder_outputs = torch.zeros(config.batch_size,config.max_len,self.vocab_size) #[seq_len,batch_size,14]
 
        decoder_hidden = encoder_hidden #[batch_size,hidden_size]
 
        for t in range(config.max_len):
            decoder_output_t , decoder_hidden = self.forward_step(decoder_input,decoder_hidden)
            # print(decoder_output_t.size(),decoder_hidden.size())
            # print(decoder_outputs.size())
            decoder_outputs[:,t,:] = decoder_output_t
 
            use_teacher_forcing = random.random() > 0.5
            if use_teacher_forcing:
                decoder_input =target[:,t].unsqueeze(1)  #[batch_size,1]
            else:
                value, index = torch.topk(decoder_output_t, 1) # index [batch_size,1]
                decoder_input = index
            # print("decoder_input size:",decoder_input.size(),use_teacher_forcing)
        return decoder_outputs,decoder_hidden
 
    def forward_step(self,decoder_input,decoder_hidden):
        """
        :param decoder_input:[batch_size,1]
        :param decoder_hidden: [1,batch_size,hidden_size]
        :return: out:[batch_size,vocab_size],decoder_hidden:[1,batch_size,didden_size]
        """
        embeded = self.embedding(decoder_input)  #embeded: [batch_size,1 , embedding_dim]
        # print("forworad step embeded:",embeded.size())
        out,decoder_hidden = self.gru(embeded,decoder_hidden) #out [1, batch_size, hidden_size]
        # print("forward_step out size:",out.size()) #[1, batch_size, hidden_size]
        out = out.squeeze(0)
        out = F.log_softmax(self.fc(out),dim=-1)#[batch_Size, vocab_size]
        out = out.squeeze(1)
        # print("out size:",out.size(),decoder_hidden.size())
        return out,decoder_hidden
 
    def evaluation(self,encoder_hidden): #[1, 20, 14]
        # target = target.transpose(0, 1)  # batch_first = False
        batch_size = encoder_hidden.size(1)
 
        decoder_input = torch.LongTensor([[num_sequence.SOS] * batch_size])
        # print("decoder start input size:",decoder_input.size()) #[1, 20]
        decoder_outputs = torch.zeros(batch_size,config.max_len, self.vocab_size)  # [seq_len,batch_size,14]
        decoder_hidden = encoder_hidden
 
        for t in range(config.max_len):
            decoder_output_t, decoder_hidden = self.forward_step(decoder_input, decoder_hidden)
            decoder_outputs[:,t,:] = decoder_output_t
            value, index = torch.topk(decoder_output_t, 1)  # index [20,1]
            decoder_input = index.transpose(0, 1)
 
        # print("decoder_outputs size:",decoder_outputs.size())
        # # 获取输出的id
        decoder_indices =[]
        # decoder_outputs = decoder_outputs.transpose(0,1) #[batch_size,seq_len,vocab_size]
        # print("decoder_outputs size",decoder_outputs.size())
        for i in range(decoder_outputs.size(1)):
            value,indices = torch.topk(decoder_outputs[:,i,:],1)
            # print("indices size",indices.size(),indices)
            # indices  = indices.transpose(0,1)
            decoder_indices.append(int(indices[0][0].data))
        return decoder_indices

 6.完成seq2seq模型(seq2seq.py)

import torch
import torch.nn as nn
class Seq2Seq(nn.Module):
    def __init__(self,encoder,decoder):
        super(Seq2Seq,self).__init__()
        self.encoder = encoder
        self.decoder = decoder
 
    def forward(self, input,target,input_length,target_length):
        encoder_outputs,encoder_hidden = self.encoder(input,input_length)
        decoder_outputs,decoder_hidden = self.decoder(encoder_hidden,target,target_length)
        return decoder_outputs,decoder_hidden
 
    def evaluation(self,inputs,input_length):
        encoder_outputs,encoder_hidden = self.encoder(inputs,input_length)
        decoded_sentence = self.decoder.evaluation(encoder_hidden)
        return decoded_sentence

7.完成训练

import torch
import config
from torch import optim
import torch.nn as nn
from encoder import NumEncoder
from decoder import NumDecoder
from seq2seq import Seq2Seq
from dataset import data_loader as train_dataloader
from word_sequence import num_sequence
from tqdm import tqdm
encoder = NumEncoder()
decoder = NumDecoder()
model = Seq2Seq(encoder,decoder)
for name, param in model.named_parameters():
    if 'bias' in name:
        torch.nn.init.constant_(param, 0.0)
    elif 'weight' in name:
        torch.nn.init.xavier_normal_(param)
optimizer =  optim.Adam(model.parameters())
criterion= nn.NLLLoss(ignore_index=num_sequence.PAD,reduction="mean")
def get_loss(decoder_outputs,target):
    target = target.view(-1) #[batch_size*max_len]
    decoder_outputs = decoder_outputs.view(config.batch_size*config.max_len,-1)
    return criterion(decoder_outputs,target)
def train(epoch):
    total_loss = 0
    correct = 0
    total = 0
    progress_bar = tqdm(total=len(train_dataloader), desc='Train Epoch {}'.format(epoch), unit='batch')
    for idx, (input, target, input_length, target_len) in enumerate(train_dataloader):
        optimizer.zero_grad()
        ##[seq_len,batch_size,vocab_size] [batch_size,seq_len]
        decoder_outputs, decoder_hidden = model(input, target, input_length, target_len)
        loss = get_loss(decoder_outputs, target)
        total_loss += loss.item()
        loss.backward()
        optimizer.step()
        _, predicted = torch.max(decoder_outputs.data, 2)
        correct += (predicted == target).sum().item()
        total += target.size(0) * target.size(1)
        acc = 100 * correct / total
        avg_loss = total_loss / (idx + 1)
        progress_bar.set_postfix({'loss': avg_loss, 'acc': '{:.2f}%'.format(acc)})
        progress_bar.update()
    progress_bar.close()
    torch.save(model.state_dict(), "models/seq2seq_model.pkl")
    torch.save(optimizer.state_dict(), 'models/seq2seq_optimizer.pkl')
if __name__ == '__main__':
    for i in range(10):
        train(i)

8.进行评估

随机生成10000个测试集进行模型的验证，然后输入一串数字观察输出结果

import torch
from encoder import NumEncoder
from decoder import NumDecoder
from seq2seq import Seq2Seq
from word_sequence import num_sequence
import random
encoder = NumEncoder()
decoder = NumDecoder()
model = Seq2Seq(encoder,decoder)
model.load_state_dict(torch.load("models/seq2seq_model.pkl"))
def evaluate():
    correct = 0
    total = 0
    for i in range(10000):
        test_words = random.randint(1,100000000)
        test_word_len = [len(str(test_words))]
        _test_words = torch.LongTensor([num_sequence.transform(test_words)])
        decoded_indices = model.evaluation(_test_words,test_word_len)
        result = num_sequence.inverse_transform(decoded_indices)
        if str(test_words)+"0" == "".join(result):
            correct += 1
        total += 1
    accuracy = correct/total
    print("10000个测试集的Acc: ", accuracy)
def predict():
    test_word = input("Enter a number to predict: ")
    test_word_len = [len(test_word)]
    _test_word = torch.LongTensor([num_sequence.transform(int(test_word))])
    decoded_indices = model.evaluation(_test_word,test_word_len)
    result = num_sequence.inverse_transform(decoded_indices)
    print("Prediction: ", "".join(result))
if __name__ == '__main__':
    evaluate()
    predict()

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