背景
经过前几篇文章的内容，基本跑通了整个文本摘要的基本流程。主要包括：文本预处理、基于注意力机制的seq2seq文本摘要生成、解码算法、模型生成结果评估等。因此，经过前面的操作，基本可以得到一个完整的文本摘要抽取的过程，本文的内容旨在对抽取过程进行进一步的提升。本文主要实现的是transformer和bert模型之前的一个较为经典的模型——Pointer Generate Network，其理论部分的内容已在其他文章交代，本文重在其代码实现部分。
核心内容
本文内容分别就数据加载、模型构建、模型训练等几个主要部分，在原来baseline的基础上进行修改，其中模型评估部分，和基于seq2seq模型的过程基本相同，因此不在叙述其具体实现。完整代码在本文最后会附上代码。
整体流程
整体流程在前面基于seq2seq和Attention的文本摘要模型中已做具体介绍，本文旨在利用新方法，对模型性能进行提升，因此整体架构基本不变，在此不在累述。只在大体流程的部分细节进行优化，例如：使用生成器进行数据加载、学习率及损失函数的改进、Attention计算时考虑mask等，具体将在代码中描述。
基于generator的数据批量加载
# 构造数据
train_dataset, params['train_steps_per_epoch'] = batcher(vocab, params)
valid_dataset, params['valid_steps_per_epoch'] = batcher(vocab, params)
logger.info(f'Building the dataset for train/valid ...')
其中,batcher为本次加载数据方法的不同之处。
def batcher(vocab, params):
dataset = batch_generator(example_generator,
params,
vocab,
params['max_enc_len'],
params['max_dec_len'],
params['batch_size'],
params['mode'],
)
dataset = dataset.prefetch(params['buffer_size'])
steps_per_epoch = get_steps_per_epoch(params)
return dataset, steps_per_epoch
example_generator为数据加载时的具体生成器，可根据数据格式，进行具体编写。而batch_generator会根据example_generator的返回的结果，构造成符合之前tensorflow读取数据的dataset样式，因此如何在example_generator中依旧保持符合tensorflow的常用工具dataset就成为核心。整体流程和过去保持一致。具体如下：
def batch_generator(generator, params, vocab, max_enc_len, max_dec_len, batch_size, mode):
dataset = tf.data.Dataset.from_generator(lambda: generator(params,
vocab,
max_enc_len,
max_dec_len,
mode,
# batch_size
),
output_types={
'enc_len': tf.int32,
'enc_input': tf.int32,
'enc_input_extend_vocab': tf.int32,
'article_oovs': tf.string,
'dec_input': tf.int32,
'target': tf.int32,
'dec_len': tf.int32,
'article': tf.string,
'abstract': tf.string,
'abstract_sents': tf.string,
'decoder_pad_mask': tf.int32,
'encoder_pad_mask': tf.int32},
output_shapes={
'enc_len': [],
'enc_input': [None],
'enc_input_extend_vocab': [None],
'article_oovs': [None],
'dec_input': [None],
'target': [None],
'dec_len': [],
'article': [],
'abstract': [],
'abstract_sents': [],
'decoder_pad_mask': [None],
'encoder_pad_mask': [None]})
dataset = dataset.padded_batch(batch_size=batch_size,
padded_shapes=({'enc_len': [],
'enc_input': [None],
'enc_input_extend_vocab': [None],
'article_oovs': [None],
'dec_input': [max_dec_len],
'target': [max_dec_len],
'dec_len': [],
'article': [],
'abstract': [],
'abstract_sents': [],
'decoder_pad_mask': [max_dec_len],
'encoder_pad_mask': [None]}),
padding_values={'enc_len': -1,
'enc_input': vocab.word2index[vocab.PAD_TOKEN],
'enc_input_extend_vocab': vocab.word2index[vocab.PAD_TOKEN],
'article_oovs': b'',
'dec_input': vocab.word2index[vocab.PAD_TOKEN],
'target': vocab.word2index[vocab.PAD_TOKEN],
'dec_len': -1,
'article': b'',
'abstract': b'',
'abstract_sents': b'',
'decoder_pad_mask': 0,
'encoder_pad_mask': 0},
drop_remainder=True)
def update(entry):
return ({
"enc_input": entry["enc_input"],
"extended_enc_input": entry["enc_input_extend_vocab"],
"article_oovs": entry["article_oovs"],
"enc_len": entry["enc_len"],
"article": entry["article"],
"max_oov_len": tf.shape(entry["article_oovs"])[1],
"encoder_pad_mask": entry["encoder_pad_mask"]
},
{
"dec_input": entry["dec_input"],
"dec_target": entry["target"],
"dec_len": entry["dec_len"],
"abstract": entry["abstract"],
"decoder_pad_mask": entry["decoder_pad_mask"]
})
dataset = dataset.map(update)
return dataset
在batch_generator中，使用tf.data.Dataset.from_generator()接口，能使使得数据利用生成器一条一条进行生成。在该方法中需要指定输出形状output_shape和输出类型output_types参数。并且，可以在生成数据的同时，进行一定的预处理，padded_batch来对齐长度信息，并将长度不够的词汇自动按照指定的值进行填充。
def example_generator(params, vocab, max_enc_len, max_dec_len, mode):
if mode != 'test':
dataset_x = tf.data.TextLineDataset(params[f'{mode}_seg_x_dir'])
dataset_y = tf.data.TextLineDataset(params[f'{mode}_seg_y_dir'])
train_dataset = tf.data.Dataset.zip((dataset_x, dataset_y)).take(count=10000)
if mode == 'train':
train_dataset = train_dataset.shuffle(10, reshuffle_each_iteration=True).repeat(1)
for raw_record in train_dataset:
start_decoding = vocab.word_to_index(vocab.START_DECODING)
stop_decoding = vocab.word_to_index(vocab.STOP_DECODING)
article = raw_record[0].numpy().decode('utf-8')
article_words = article.split()[:max_enc_len]
enc_input = [vocab.word_to_index(w) for w in article_words]
enc_input_extend_vocab, article_oovs = article_to_index(article_words, vocab)
# add start and stop flag
enc_input = get_enc_inp_targ_seqs(enc_input,
max_enc_len,
start_decoding,
stop_decoding)
enc_input_extend_vocab = get_enc_inp_targ_seqs(enc_input_extend_vocab,
max_enc_len,
start_decoding,
stop_decoding)
# mark长度
enc_len = len(enc_input)
# 添加mark标记
encoder_pad_mask = [1 for _ in range(enc_len)]
abstract = raw_record[1].numpy().decode('utf-8')
abstract_words = abstract.split()
abs_ids = [vocab.word_to_index(w) for w in abstract_words]
dec_input, target = get_dec_inp_targ_seqs(abs_ids,
max_dec_len,
start_decoding,
stop_decoding)
if params['pointer_gen']:
abs_ids_extend_vocab = abstract_to_index(abstract_words, vocab, article_oovs)
_, target = get_dec_inp_targ_seqs(abs_ids_extend_vocab,
max_dec_len,
start_decoding,
stop_decoding)
# mark长度
dec_len = len(target)
# 添加mark标记
decoder_pad_mask = [1 for _ in range(dec_len)]
output = {
"enc_len": enc_len,
"enc_input": enc_input,
"enc_input_extend_vocab": enc_input_extend_vocab,
"article_oovs": article_oovs,
"dec_input": dec_input,
"target": target,
"dec_len": dec_len,
"article": article,
"abstract": abstract,
"abstract_sents": abstract,
"decoder_pad_mask": decoder_pad_mask,
"encoder_pad_mask": encoder_pad_mask
}
yield output
else:
test_dataset = tf.data.TextLineDataset(params['valid_seg_x_dir'])
for raw_record in test_dataset:
article = raw_record.numpy().decode('utf-8')
article_words = article.split()[: max_enc_len]
enc_len = len(article_words)
enc_input = [vocab.word_to_index(w) for w in article_words]
enc_input_extend_vocab, article_oovs = article_to_index(article_words, vocab)
# 添加mark标记
encoder_pad_mask = [1 for _ in range(enc_len)]
output = {
"enc_len": enc_len,
"enc_input": enc_input,
"enc_input_extend_vocab": enc_input_extend_vocab,
"article_oovs": article_oovs,
"dec_input": [],
"target": [],
"dec_len": params['max_dec_len'],
"article": article,
"abstract": '',
"abstract_sents": '',
"decoder_pad_mask": [],
"encoder_pad_mask": encoder_pad_mask
}
# 每一批的数据都一样阿, 是的是为了beam search
if params["decode_mode"] == "beam":
for _ in range(params["batch_size"]):
yield output
elif params["decode_mode"] == "greedy":
yield output
else:
print("shit")
在example_generator函数中，就是对具体数据的处理。其中包括：添加开始、结束标志(
PGN模型中另一个重要的点在于，其最终预测的概率分布是 词汇表长度的概率分布 + 输出数据(input text)的attention 的分布。(个人理解，同时出现在 词汇表 和input text的词的概率会被增大；按照贪心解码取概率的最大的1个或者K个词的思路，得到原始词汇表以外的词概率较小，因此该模型只能在一定程度上缓解oov问题。）
代码中实现上述两个内容的基础时在对数据进行处理时，函数article_to_index和abstract_to_index两个函数中，将原来标注为UNK的单词，重新标注为出现在输入数据中的词（即较少了UNK）。
def article_to_index(article_words, vocab):
oov_words = []
extend_vocab_index = []
unk_index = vocab.UNKNOWN_TOKEN_INDEX
for word in article_words:
word_index = vocab.word_to_index(word)
if word_index == unk_index:
if word not in oov_words:
oov_words.append(word)
oov_num = oov_words.index(word)
extend_vocab_index.append(vocab.size() + oov_num)
else:
extend_vocab_index.append(word_index)
return extend_vocab_index, oov_words
将原来的词汇表进行扩展的代码样例如上，这里只展示了对训练数据X的处理，对于y的处理类似，后续完整代码可见。
数据保存部分的代码优化
# 构造模型保存管理器
checkpoint = tf.train.Checkpoint(PGN=model)
checkpoint_manager = tf.train.CheckpointManager(checkpoint, params['checkpoint_dir'], max_to_keep=5)
if checkpoint_manager.latest_checkpoint:
checkpoint_manager.restore(checkpoint_manager.latest_checkpoint)
params['trained_epoch'] = int(checkpoint_manager.latest_checkpoint[-1])
logger.info(f'Building model by restoring {checkpoint_manager.latest_checkpoint}')
else:
params['trained_epoch'] = 1
logger.info('Building model from initial ...')
# 设置学习率
params['learning_rate'] *= np.power(0.95, params['trained_epoch'])
logger.info(f'Learning rate : {params["learning_rate"]}')
上述代码的优化点在于对动态学习率的设置、自动加载上一次训练保存的最优模型，以及训练了多少epoch。
PGN模型构建
class PGN(keras.Model):
def __init__(self, params):
super(PGN, self).__init__()
self.embedding_matrix = load_embedding_matrix(max_vocab_size=params['vocab_size'])
self.vocab_size = params['vocab_size']
self.batch_size = params['batch_size']
self.encoder = Encoder(self.embedding_matrix,
params['enc_units'],
params['batch_size'])
self.decoder = Decoder(self.embedding_matrix,
params['dec_units'],
params['batch_size'])
self.pointer = Pointer()
def call_one_step(self, dec_input, dec_hidden, enc_output, enc_pad_mask, use_coverage, prev_coverage):
context_vector, dec_hidden, dec_x, prediction, attention_weights, coverage = self.decoder(dec_input,
dec_hidden,
enc_output,
enc_pad_mask,
prev_coverage,
use_coverage)
p_gens = self.pointer(context_vector, dec_hidden, dec_x)
return prediction, dec_hidden, context_vector, attention_weights, p_gens, coverage
def call(self, dec_input, dec_hidden, enc_output, enc_extended_input, batch_oov_len, enc_pad_mask, use_coverage,
coverage=None):
predictions = []
attentions = []
p_gens = []
coverages = []
for t in range(dec_input.shape[1]):
final_dists, dec_hidden, context_vector, attention_weights, p_gen, coverage = self.call_one_step(
dec_input[:, t],
dec_hidden,
enc_output,
enc_pad_mask,
use_coverage,
coverage)
coverages.append(coverage)
predictions.append(final_dists)
attentions.append(attention_weights)
p_gens.append(p_gen)
final_dists = _calc_final_dist(enc_extended_input,
predictions,
attentions,
p_gens,
batch_oov_len,
self.vocab_size,
self.batch_size)
attentions = tf.stack(attentions, axis=1)
return tf.stack(final_dists, 1), attentions, tf.stack(coverage, 1)
def _calc_final_dist(_enc_batch_extend_vocab, vocab_dists, attn_dists, p_gens, batch_oov_len, vocab_size, batch_size):
"""
Calculate the final distribution, for the pointer-generator model
Args:
vocab_dists: The vocabulary distributions. List length max_dec_steps of (batch_size, vsize) arrays.
The words are in the order they appear in the vocabulary file.
attn_dists: The attention distributions. List length max_dec_steps of (batch_size, attn_len) arrays
Returns:
final_dists: The final distributions. List length max_dec_steps of (batch_size, extended_vsize) arrays.
"""
# Multiply vocab dists by p_gen and attention dists by (1-p_gen)
vocab_dists = [p_gen * dist for (p_gen, dist) in zip(p_gens, vocab_dists)]
attn_dists = [(1 - p_gen) * dist for (p_gen, dist) in zip(p_gens, attn_dists)]
# Concatenate some zeros to each vocabulary dist, to hold the probabilities for in-article OOV words
extended_vsize = vocab_size + batch_oov_len # the maximum (over the batch) size of the extended vocabulary
extra_zeros = tf.zeros((batch_size, batch_oov_len))
# list length max_dec_steps of shape (batch_size, extended_vsize)
vocab_dists_extended = [tf.concat(axis=1, values=[dist, extra_zeros]) for dist in vocab_dists]
# Project the values in the attention distributions onto the appropriate entries in the final distributions
# This means that if a_i = 0.1 and the ith encoder word is w, and w has index 500 in the vocabulary,
# then we add 0.1 onto the 500th entry of the final distribution
# This is done for each decoder timestep.
# This is fiddly; we use tf.scatter_nd to do the projection
batch_nums = tf.range(0, limit=batch_size) # shape (batch_size)
batch_nums = tf.expand_dims(batch_nums, 1) # shape (batch_size, 1)
attn_len = tf.shape(_enc_batch_extend_vocab)[1] # number of states we attend over
batch_nums = tf.tile(batch_nums, [1, attn_len]) # shape (batch_size, attn_len)
indices = tf.stack((batch_nums, _enc_batch_extend_vocab), axis=2) # shape (batch_size, enc_t, 2)
shape = [batch_size, extended_vsize]
# list length max_dec_steps (batch_size, extended_vsize)
attn_dists_projected = [tf.scatter_nd(indices, copy_dist, shape) for copy_dist in attn_dists]
# Add the vocab distributions and the copy distributions together to get the final distributions
# final_dists is a list length max_dec_steps; each entry is a tensor shape (batch_size, extended_vsize) giving
# the final distribution for that decoder timestep
# Note that for decoder timesteps and examples corresponding to a [PAD] token, this is junk - ignore.
final_dists = [vocab_dist + copy_dist for (vocab_dist, copy_dist) in
zip(vocab_dists_extended, attn_dists_projected)]
return final_dists
PGN模型的构成主要有以下几部分：encoder、decoder、gen_pointer。本文中的实现，采用Teacher Forcing进行单词的生成过程。在函数_calc_final_dist()中，用于计算最终扩展后的概率分布。call_one_step()函数用于call调用，每次生成一个解码的词。下面将继续介绍模型的几个组件。
class Encoder(keras.Model):
def __init__(self, embedding_matrix, enc_units, batch_size):
super(Encoder, self).__init__()
self.batch_size = batch_size
self.enc_units = enc_units
self.vocab_size, self.embedding_dim = embedding_matrix.shape
self.embedding = keras.layers.Embedding(self.vocab_size,
self.embedding_dim,
weights=[embedding_matrix],
trainable=False)
self.gru = keras.layers.GRU(self.enc_units,
return_state=True,
return_sequences=True,
recurrent_initializer='glorot_uniform')
self.bidirectional_gru = keras.layers.Bidirectional(self.gru)
def call(self, x, enc_hidden):
x = self.embedding(x) # x shape: batch_size * enc_units -> batch_size * 128
# enc_output shape: batch * max_len * enc_unit
enc_output, forward_state, backward_state = self.bidirectional_gru(x, initial_state=[enc_hidden, enc_hidden])
# enc_hidden shape: batch_size * 256
enc_hidden = keras.layers.concatenate([forward_state, backward_state], axis=-1)
return enc_output, enc_hidden
def initialize_hidden_state(self):
return tf.zeros(shape=(self.batch_size, self.enc_units))
encoder部分和前面基于seq2seq模型基本相同，差别在于此处使用了一个双向的gru进行编码表示。
def masked_attention(enc_pad_mask, attn_dist):
attn_dist = tf.squeeze(attn_dist, axis=2)
mask = tf.cast(enc_pad_mask, dtype=attn_dist.dtype)
attn_dist *= mask
mask_sum = tf.reduce_sum(attn_dist, axis=1)
attn_dist = attn_dist / tf.reshape(mask_sum + 1e-12, [-1, 1])
attn_dist = tf.expand_dims(attn_dist, axis=2)
return attn_dist
class BahdanauAttention(keras.layers.Layer):
def __init__(self, units):
super(BahdanauAttention, self).__init__()
self.W_s = keras.layers.Dense(units)
self.W_h = keras.layers.Dense(units)
self.W_c = keras.layers.Dense(units)
self.V = keras.layers.Dense(1)
def call(self, dec_hidden, enc_output, enc_pad_mask, use_coverage=False, pre_coverage=None):
hidden_with_time_axis = tf.expand_dims(dec_hidden, 1)
if use_coverage and pre_coverage is not None:
score = self.V(tf.nn.tanh(self.W_s(enc_output) + self.W_h(hidden_with_time_axis) + self.W_c(pre_coverage)))
attention_weights = tf.nn.softmax(score, axis=1)
attention_weights = masked_attention(enc_pad_mask, attention_weights)
coverage = attention_weights + pre_coverage
else:
score = self.V(tf.nn.tanh(self.W_s(enc_output) + self.W_h(hidden_with_time_axis)))
attention_weights = tf.nn.softmax(score)
attention_weights = masked_attention(enc_pad_mask, attention_weights)
if use_coverage:
coverage = attention_weights
else:
coverage = []
context_vactor = attention_weights * enc_output
context_vactor = tf.reduce_sum(context_vactor, axis=1)
return context_vactor, tf.squeeze(attention_weights, -1), coverage
Attention的计算一般可分为三个步骤：1.计算Attenrion score 2. softmax 3. reduce_sum。此处Attention过程稍微和之前的实现有所不同。主要体现在：1.计算softmax时，考虑了mask部分。在masked_attention函数中计算attention的时候，被mask的部分不参与到计算。2.考虑前一次计算attention时得到的向量。通过指定use_coverage参数指定是否使用收敛机制。
class Decoder(keras.Model):
def __init__(self, embedding_matrix, dec_units, batch_size):
super(Decoder, self).__init__()
self.batch_size = batch_size
self.dec_units = dec_units
self.vocab_size, self.embedding_dim = embedding_matrix.shape
self.embedding = keras.layers.Embedding(self.vocab_size,
self.embedding_dim,
weights=[embedding_matrix],
trainable=False)
self.cell = keras.layers.GRUCell(units=self.dec_units, recurrent_initializer='glorot_uniform')
self.fc = keras.layers.Dense(self.vocab_size, activation=keras.activations.softmax)
self.attention = BahdanauAttention(self.dec_units)
def call(self, dec_input, dec_hidden, enc_output, enc_pad_mask, pre_coverage, use_covarage=True):
dec_x = self.embedding(dec_input)
dec_output, [dec_hidden] = self.cell(dec_x, [dec_hidden])
context_vector, attention_weights, coverage = self.attention(dec_hidden,
enc_output,
enc_pad_mask,
use_covarage,
pre_coverage)
dec_output = tf.concat([dec_output, context_vector], axis=-1)
prediction = self.fc(dec_output)
return context_vector, dec_hidden, dec_x, prediction, attention_weights, coverage
decoder的部分一个比较重要的点在于，解码时是根据时间步(timestpes)一步一步进行解码，上一个时间步的输出和attention向量，会参与到下一步attention向量的计算，因此此处采用cell级别的lstm实现。
class Pointer(keras.layers.Layer):
def __init__(self):
super(Pointer, self).__init__()
self.w_s_reduce = keras.layers.Dense(1)
self.w_i_reduce = keras.layers.Dense(1)
self.w_c_reduce = keras.layers.Dense(1)
def call(self, context_vector, dec_hidden, dec_inp):
return tf.nn.sigmoid(self.w_s_reduce(dec_hidden) +
self.w_c_reduce(context_vector) +
self.w_i_reduce(dec_inp))
根据PGN模型的理论，在计算最终概率分布时，采用两个概率分布加权进行最终概率分布计算，而以多大概率进行加权呢？Pointer类主要用于计算这个值。到此为止，模型部分的细节基本介绍完毕。
模型训练以及评估
def train_model(model, train_dataset, valid_dataset, params, checkpoint_manager):
epochs = params['epochs']
optimizer = keras.optimizers.Adagrad(learning_rate=params['learning_rate'],
initial_accumulator_value=params['adagrad_init_acc'],
clipnorm=params['max_grad_norm'],
epsilon=params['eps'])
best_loss = 100
for epoch in range(epochs):
start = time.time()
enc_hidden = model.encoder.initialize_hidden_state()
total_loss = 0.
total_log_loss = 0.
total_cov_loss = 0.
step = 0
for encoder_batch_data, decoder_batch_data in train_dataset:
batch_loss, log_loss, cov_loss = train_step(model,
enc_hidden,
encoder_batch_data['enc_input'],
encoder_batch_data['extended_enc_input'],
encoder_batch_data['max_oov_len'],
decoder_batch_data['dec_input'],
decoder_batch_data['dec_target'],
enc_pad_mask=encoder_batch_data['encoder_pad_mask'],
dec_pad_mask=decoder_batch_data['decoder_pad_mask'],
params=params,
optimizer=optimizer,
mode='train')
step += 1
total_loss += batch_loss
total_log_loss += log_loss
total_cov_loss += cov_loss
if step % 50 == 0:
if params['use_coverage']:
print('Epoch {} Batch {} avg_loss {:.4f} log_loss {:.4f} cov_loss {:.4f}'.format(epoch + 1,
step,
total_loss / step,
total_log_loss / step,
total_cov_loss / step))
else:
print('Epoch {} Batch {} avg_loss {:.4f}'.format(epoch + 1,
step,
total_loss / step))
valid_total_loss, valid_total_cov_loss, valic_total_log_loss = evaluate(model, valid_dataset, params)
print('Epoch {} Loss {:.4f}, valid Loss {:.4f}'.format(epoch + 1, total_loss / step, valid_total_loss))
print('Time taken for 1 epoch {} sec\n'.format(time.time() - start))
if valid_total_loss < best_loss:
best_loss = valid_total_loss
ckpt_save_path = checkpoint_manager.save()
print('Saving checkpoint for epoch {} at {}, best valid loss {}'.format(epoch + 1,
ckpt_save_path,
best_loss))
def train_step(model, enc_hidden, enc_input, extend_enc_input, max_oov_len, dec_input, dec_target, enc_pad_mask, dec_pad_mask, params, optimizer=None, mode='train'):
with tf.GradientTape() as tape:
# encoder，逐个预测
enc_output, enc_hidden = model.encoder(enc_input, enc_hidden)
# decoder
dec_hidden = enc_hidden
final_dists, attentions, coverages = model(dec_input, dec_hidden, enc_output, extend_enc_input, max_oov_len, enc_pad_mask=enc_pad_mask, use_coverage=params['use_coverage'], coverage=None)
batch_loss, log_loss, cov_loss = calc_loss(dec_target, final_dists, dec_pad_mask, attentions, params['cov_loss_wt'], params['eps'])
if mode == 'train':
variables = (model.encoder.trainable_variables + model.decoder.trainable_variables + model.pointer.trainable_variables)
gradients = tape.gradient(batch_loss, variables)
optimizer.apply_gradients(zip(gradients, variables))
return batch_loss, log_loss, cov_loss
训练过程的整理框架基本维持不变。差别在于计算损失函数的计算。
def calc_loss(real, pred, dec_mask, attentions, cov_loss_wt, eps):
log_loss = pgn_log_loss_function(real, pred, dec_mask, eps)
cov_loss = _coverage_loss(attentions, dec_mask)
return log_loss + cov_loss_wt * cov_loss, log_loss, cov_loss
损失函数的计算包括两个部分：一个是pgn模型原有的损失，一个是使用coverage机制时带来的损失。并将最终损失做一个加权。
def pgn_log_loss_function(real, final_dists, padding_mask, eps):
loss_per_step = []
batch_nums = tf.range(0, limit=real.shape[0])
final_dists = tf.transpose(final_dists, perm=[1, 0, 2])
for dec_step, dist in enumerate(final_dists):
targets = real[:, dec_step]
indices = tf.stack((batch_nums, targets), axis=1)
gold_probs = tf.gather_nd(dist, indices)
losses = -tf.math.log(gold_probs + eps)
loss_per_step.append(losses)
_loss = _mask_and_avg(loss_per_step, padding_mask)
return _loss
def _coverage_loss(attn_dists, padding_mask):
attn_dists = tf.transpose(attn_dists, perm=[1, 0, 2])
coverage = tf.zeros_like(attn_dists[0])
covlosses = []
for a in attn_dists:
covloss = tf.reduce_sum(tf.minimum(a, coverage), [1])
covlosses.append(covloss)
coverage += a
coverage_loss = _mask_and_avg(covlosses, padding_mask)
return coverage_loss
def _mask_and_avg(values, padding_mask):
padding_mask = tf.cast(padding_mask, dtype=values[0].dtype)
dec_lens = tf.reduce_sum(padding_mask, axis=1)
values_per_step = [v * padding_mask[:, dec_step] for dec_step, v in enumerate(values)]
values_per_ex = s文本摘要-05-性能提升之PGN模型.mdum(values_per_step) / dec_lens
return tf.reduce_mean(values_per_ex)