PyTorch 高级实战教程：基于 Bi-LSTM CRF 实现命名实体识别和中文分词

前言：实测 PyTorch 代码非常简洁易懂，只需要将中文分词的数据集预处理成作者提到的格式，即可很快的就迁移了这个代码到中文分词中，相关的代码后续将会分享。

具体的数据格式，这种方式并不适合处理很多的数据，但是对于 demo 来说非常友好，把英文改成中文，标签改成分词问题中的 “ BEMS ” 就可以跑起来了。

# Make up some training data training_data = [( "the wall street journal reported today that apple corporation made money".split(), "B I I I O O O B I O O".split() ), ( "georgia tech is a university in georgia".split(), "B I O O O O B".split() )] 

Pytorch 是一个动态神经网络工具包。 动态工具包的另一个例子是 Dynet （我之所以提到这一点，因为与 Pytorch 和 Dynet 的工作方式类似。如果你在 Dynet 中看到一个例子，它可能会帮助你在 Pytorch 中实现它）。 相反的是静态工具包，包括 Theano，Keras，TensorFlow 等。核心区别如下：

在静态工具箱中，您可以定义一次计算图，对其进行编译，然后将实例流式传输给它。 在动态工具包中，您可以为每个实例定义计算图。 它永远不会被编译并且是即时执行的。 动态工具包还有一个优点，那就是更容易调试，代码更像主机语言（我的意思是 pytorch 和 dynet 看起来更像实际的 python 代码，而不是 keras 或 theano ）。

Bi-LSTM Conditional Random Field （ Bi-LSTM CRF ） 对于本节，我们将看到用于命名实体识别的 Bi-LSTM 条件随机场的完整复杂示例。 上面的 LSTM 标记符通常足以用词性标注，但是像 CRF 这样的序列模型对于 NER 上的强大性能非常重要。 假设熟悉 CRF。 虽然这个名字听起来很可怕，但所有模型都是 CRF，但是 LSTM 提供了特征。 这是一个高级模型，比本教程中的任何早期模型复杂得多。

实现细节： 下面的例子在 log 空间中实现了计算微分函数的正向算法，以及要解码的维特比算法。反向传播将自动为我们计算梯度。我们不必用手做任何事。

这个算法用来演示，没有优化。如果您了解正在发生的事情，您可能会很快看到，在转发算法中迭代下一个标记可能是在一个大型操作中完成的。我想用代码来提高可读性。如果你想做相关的改变，你可以用这个标记器来完成真正的任务。

# Author: Robert Guthrie import torch import torch.autograd as autograd import torch.nn as nn import torch.optim as optim torch.manual_seed(1) 

帮助程序函数，使代码更具可读性。

def argmax(vec): # return the argmax as a python int _, idx = torch.max(vec, 1) return idx.item() def prepare_sequence(seq, to_ix): idxs = [to_ix[w] for w in seq] return torch.tensor(idxs, dtype=torch.long) # Compute log sum exp in a numerically stable way for the forward algorithm def log_sum_exp(vec): max_score = vec[0, argmax(vec)] max_score_broadcast = max_score.view(1, -1).expand(1, vec.size()[1]) return max_score + \ torch.log(torch.sum(torch.exp(vec - max_score_broadcast))) 

创建模型

class BiLSTM_CRF(nn.Module): def __init__(self, vocab_size, tag_to_ix, embedding_dim, hidden_dim): super(BiLSTM_CRF, self).__init__() self.embedding_dim = embedding_dim self.hidden_dim = hidden_dim self.vocab_size = vocab_size self.tag_to_ix = tag_to_ix self.tagset_size = len(tag_to_ix) self.word_embeds = nn.Embedding(vocab_size, embedding_dim) self.lstm = nn.LSTM(embedding_dim, hidden_dim // 2, num_layers=1, bidirectiOnal=True) # Maps the output of the LSTM into tag space. self.hidden2tag = nn.Linear(hidden_dim, self.tagset_size) # Matrix of transition parameters. Entry i,j is the score of # transitioning *to* i *from* j. self.transitiOns= nn.Parameter( torch.randn(self.tagset_size, self.tagset_size)) # These two statements enforce the constraint that we never transfer # to the start tag and we never transfer from the stop tag self.transitions.data[tag_to_ix[START_TAG], :] = -10000 self.transitions.data[:, tag_to_ix[STOP_TAG]] = -10000 self.hidden = self.init_hidden() def init_hidden(self): return (torch.randn(2, 1, self.hidden_dim // 2), torch.randn(2, 1, self.hidden_dim // 2)) def _forward_alg(self, feats): # Do the forward algorithm to compute the partition function init_alphas = torch.full((1, self.tagset_size), -10000.) # START_TAG has all of the score. init_alphas[0][self.tag_to_ix[START_TAG]] = 0. # Wrap in a variable so that we will get automatic backprop forward_var = init_alphas # Iterate through the sentence for feat in feats: alphas_t = [] # The forward tensors at this timestep for next_tag in range(self.tagset_size): # broadcast the emission score: it is the same regardless of # the previous tag emit_score = feat[next_tag].view( 1, -1).expand(1, self.tagset_size) # the ith entry of trans_score is the score of transitioning to # next_tag from i trans_score = self.transitions[next_tag].view(1, -1) # The ith entry of next_tag_var is the value for the # edge (i -> next_tag) before we do log-sum-exp next_tag_var = forward_var + trans_score + emit_score # The forward variable for this tag is log-sum-exp of all the # scores. alphas_t.append(log_sum_exp(next_tag_var).view(1)) forward_var = torch.cat(alphas_t).view(1, -1) terminal_var = forward_var + self.transitions[self.tag_to_ix[STOP_TAG]] alpha = log_sum_exp(terminal_var) return alpha def _get_lstm_features(self, sentence): self.hidden = self.init_hidden() embeds = self.word_embeds(sentence).view(len(sentence), 1, -1) lstm_out, self.hidden = self.lstm(embeds, self.hidden) lstm_out = lstm_out.view(len(sentence), self.hidden_dim) lstm_feats = self.hidden2tag(lstm_out) return lstm_feats def _score_sentence(self, feats, tags): # Gives the score of a provided tag sequence score = torch.zeros(1) tags = torch.cat([torch.tensor([self.tag_to_ix[START_TAG]], dtype=torch.long), tags]) for i, feat in enumerate(feats): score = score + \ self.transitions[tags[i + 1], tags[i]] + feat[tags[i + 1]] score = score + self.transitions[self.tag_to_ix[STOP_TAG], tags[-1]] return score def _viterbi_decode(self, feats): backpointers = [] # Initialize the viterbi variables in log space init_vvars = torch.full((1, self.tagset_size), -10000.) init_vvars[0][self.tag_to_ix[START_TAG]] = 0 # forward_var at step i holds the viterbi variables for step i-1 forward_var = init_vvars for feat in feats: bptrs_t = [] # holds the backpointers for this step viterbivars_t = [] # holds the viterbi variables for this step for next_tag in range(self.tagset_size): # next_tag_var[i] holds the viterbi variable for tag i at the # previous step, plus the score of transitioning # from tag i to next_tag. # We don't include the emission scores here because the max # does not depend on them (we add them in below) next_tag_var = forward_var + self.transitions[next_tag] best_tag_id = argmax(next_tag_var) bptrs_t.append(best_tag_id) viterbivars_t.append(next_tag_var[0][best_tag_id].view(1)) # Now add in the emission scores, and assign forward_var to the set # of viterbi variables we just computed forward_var = (torch.cat(viterbivars_t) + feat).view(1, -1) backpointers.append(bptrs_t) # Transition to STOP_TAG terminal_var = forward_var + self.transitions[self.tag_to_ix[STOP_TAG]] best_tag_id = argmax(terminal_var) path_score = terminal_var[0][best_tag_id] # Follow the back pointers to decode the best path. best_path = [best_tag_id] for bptrs_t in reversed(backpointers): best_tag_id = bptrs_t[best_tag_id] best_path.append(best_tag_id) # Pop off the start tag (we dont want to return that to the caller) start = best_path.pop() assert start == self.tag_to_ix[START_TAG] # Sanity check best_path.reverse() return path_score, best_path def neg_log_likelihood(self, sentence, tags): feats = self._get_lstm_features(sentence) forward_score = self._forward_alg(feats) gold_score = self._score_sentence(feats, tags) return forward_score - gold_score def forward(self, sentence): # dont confuse this with _forward_alg above. # Get the emission scores from the BiLSTM lstm_feats = self._get_lstm_features(sentence) # Find the best path, given the features. score, tag_seq = self._viterbi_decode(lstm_feats) return score, tag_seq 

开始训练

START_TAG = "<START>" STOP_TAG = "<STOP>" EMBEDDING_DIM = 5 HIDDEN_DIM = 4 # Make up some training data training_data = [( "the wall street journal reported today that apple corporation made money".split(), "B I I I O O O B I O O".split() ), ( "georgia tech is a university in georgia".split(), "B I O O O O B".split() )] word_to_ix = {} for sentence, tags in training_data: for word in sentence: if word not in word_to_ix: word_to_ix[word] = len(word_to_ix) tag_to_ix = {"B": 0, "I": 1, "O": 2, START_TAG: 3, STOP_TAG: 4} model = BiLSTM_CRF(len(word_to_ix), tag_to_ix, EMBEDDING_DIM, HIDDEN_DIM) optimizer = optim.SGD(model.parameters(), lr=0.01, weight_decay=1e-4) # Check predictions before training with torch.no_grad(): precheck_sent = prepare_sequence(training_data[0][0], word_to_ix) precheck_tags = torch.tensor([tag_to_ix[t] for t in training_data[0][1]], dtype=torch.long) print(model(precheck_sent)) # Make sure prepare_sequence from earlier in the LSTM section is loaded for epoch in range( 300): # again, normally you would NOT do 300 epochs, it is toy data for sentence, tags in training_data: # Step 1. Remember that Pytorch accumulates gradients. # We need to clear them out before each instance model.zero_grad() # Step 2. Get our inputs ready for the network, that is, # turn them into Tensors of word indices. sentence_in = prepare_sequence(sentence, word_to_ix) targets = torch.tensor([tag_to_ix[t] for t in tags], dtype=torch.long) # Step 3. Run our forward pass. loss = model.neg_log_likelihood(sentence_in, targets) # Step 4. Compute the loss, gradients, and update the parameters by # calling optimizer.step() loss.backward() optimizer.step() # Check predictions after training with torch.no_grad(): precheck_sent = prepare_sequence(training_data[0][0], word_to_ix) print(model(precheck_sent)) # We got it! 

输出

(tensor(2.6907), [1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1]) (tensor(20.4906), [0, 1, 1, 1, 2, 2, 2, 0, 1, 2, 2]) 

我们没有必要在进行解码时创建计算图，因为我们不会从维特比路径得分反向传播。 因为无论如何我们都有它，尝试训练标记器，其中损失函数是维特比路径得分和测试标准路径得分之间的差异。 应该清楚的是，当预测的标签序列是正确的标签序列时，该功能是非负的和 0。 这基本上是结构感知器。

由于已经实现了 Viterbi 和 score_sentence，因此这种修改应该很短。 这是取决于训练实例的计算图形的示例。 虽然我没有尝试在静态工具包中实现它，但我想它可能但不那么直截了当。

拿起一些真实数据并进行比较！

原文链接： https://pytorch.org/tutorials/beginner/nlp/advanced_tutorial.html#advanced-making-dynamic-decisions-and-the-bi-lstm-crf

更多 PyTorch 实战教程： http://pytorchchina.com/