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model.py
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import torch
import torch.nn as nn
START_TAG = "<START>"
STOP_TAG = "<STOP>"
def argmax(vec):
_, idx = torch.max(vec, 1)
return idx.item()
def prepare_sequence(seq, to_ix):
idxs = map(lambda x: to_ix[x.lower()] if x in to_ix else to_ix['<unk>'], seq)
return torch.tensor(idxs, dtype=torch.long)
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)))
class BiLSTM_CRF(nn.Module):
def __init__(self, vocab_size, tag_to_ix, embedding_dim, hidden_dim, tag_count, tag_embedding_size):
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.pos_embeds = nn.Embedding(tag_count, tag_embedding_size)
self.lstm = nn.LSTM(embedding_dim + tag_embedding_size, hidden_dim // 2,
num_layers=2, bidirectional=True, dropout=0.2)
self.hidden2tag = nn.Linear(hidden_dim, self.tagset_size)
self.transitions = nn.Parameter(
torch.randn(self.tagset_size, self.tagset_size))
self.transitions.data[tag_to_ix[START_TAG], :] = -10000
self.transitions.data[:, tag_to_ix[STOP_TAG]] = -10000
self.transitions.data[2][0] = -100000
self.transitions.data[1][1] = -100000
self.transitions.data[2][tag_to_ix[START_TAG]] = -100000
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):
init_alphas = torch.full((1, self.tagset_size), -10000.)
init_alphas[0][self.tag_to_ix[START_TAG]] = 0.
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):
emit_score = feat[next_tag].view(
1, -1).expand(1, self.tagset_size)
trans_score = self.transitions[next_tag].view(1, -1)
next_tag_var = forward_var + trans_score + emit_score
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, sentence_pos):
self.hidden = self.init_hidden()
embeds = self.word_embeds(sentence).view(len(sentence), 1, -1)
pos_embeds_out = self.pos_embeds(sentence_pos).view(len(sentence), 1, -1)
tot_embedding = torch.cat((embeds, pos_embeds_out), dim=2)
# lstm_out, self.hidden = self.lstm(tot_embedding, self.hidden)
# TODO: I removed self.hidden to add 2 layers
lstm_out, self.hidden = self.lstm(tot_embedding)
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):
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 = []
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 = 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))
forward_var = (torch.cat(viterbivars_t) + feat).view(1, -1)
backpointers.append(bptrs_t)
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]
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)
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, sentence_pos, tags):
feats = self._get_lstm_features(sentence, sentence_pos)
forward_score = self._forward_alg(feats)
gold_score = self._score_sentence(feats, tags)
return forward_score - gold_score
def forward(self, sentence, sentence_pos):
lstm_feats = self._get_lstm_features(sentence, sentence_pos)
score, tag_seq = self._viterbi_decode(lstm_feats)
return score, tag_seq