构建文本摘要Baseline并且开始训练

基于前面word2vec的原理与训练实践、seq2seq模型的原理与实践以及attention机制，已经分别写了相关的文章来记录，此篇文章就是基于前面所学，开始着手训练文本摘要模型，当然仅是一个比较普通的baseline，后面还会不断优化模型。

构建seq2seq模型

首先利用上一节seq2seq实践中，封装的encoder、decoder和attention，集成到此模型中来，另外就是增加了一个训练技巧–teacher forcing。那么teacher forcing是啥意思呢？

seq2seq模型的输出为decoder解码出的一系列概率分布，因此采用何种方式进行解码，就显得尤为重要。如贪心解码(greedy search)、teacher forcing以及介于两种之间的beam search等。
    贪心解码的思想是，预测 t 时刻输出的单词时，直接将t−1时刻的输出词汇表中概率最大的单词，作为t时刻的输入，因此可能导致如果前一个预测值就不准的话，后面一系列都不准的问题。
    Teacher Forcing的方法是，预测 t时刻输出的单词时，直接将t−1时刻的实际单词，作为输入，因此可能带来的问题是，训练过程预测良好（因为有标签，即实际单词），但是测试过程极差（因为测试过程不会给对应的真实单词）。
    实际应用中，往往采用介于这两种极端方式之间的解码方式，如beam search 等，具体思路是预测 t 时刻输出的单词时，保留t−1时刻的输出词汇表中概率最大的前K个单词，以此带来更多的可能性（解决第一个方法的缺陷）；而且在训练过程，采用一定的概率P,来决定是否使用真实单词作为输入（解决第二个方法的缺陷）。greedy search 和beam search后面我们也会一一介绍，下面是teacher forcing的具体实现。

import tensorflow as tf

from src.seq2seq_tf2.model_layers import Encoder, BahdanauAttention, Decoder
from src.utils.gpu_utils import config_gpu
from src.utils.params_utils import get_params
from src.utils.wv_loader import load_embedding_matrix, Vocab


class Seq2Seq(tf.keras.Model):
    def __init__(self, params, vocab):
        super(Seq2Seq, self).__init__()
        self.embedding_matrix = load_embedding_matrix()
        self.params = params
        self.vocab = vocab
        self.batch_size = params["batch_size"]
        self.enc_units = params["enc_units"]
        self.dec_units = params["dec_units"]
        self.attn_units = params["attn_units"]
        self.encoder = Encoder(self.embedding_matrix,
                               self.enc_units,
                               self.batch_size)

        self.attention = BahdanauAttention(self.attn_units)

        self.decoder = Decoder(self.embedding_matrix,
                               self.dec_units,
                               self.batch_size)

    def teacher_decoder(self, dec_hidden, enc_output, dec_target):
        predictions = []

        # 第一个输入<START>
        dec_input = tf.expand_dims([self.vocab.START_DECODING_INDEX] * self.batch_size, 1)

        #  Teacher forcing 将target作为下一次的输入，依次解码
        for t in range(1, dec_target.shape[1]):
            # passing enc_output to the decoder
            # 应用decoder来一步一步预测生成词语概论分布
            pred, dec_hidden, _ = self.decoder(dec_input, dec_hidden, enc_output)
            dec_input = tf.expand_dims(dec_target[:, t], 1)

            predictions.append(pred)

        return tf.stack(predictions, 1), dec_hidden

开始训练

import tensorflow as tf

from src.seq2seq_tf2.seq2seq_model import Seq2Seq
from src.seq2seq_tf2.train_helper import train_model
from src.utils.gpu_utils import config_gpu
from src.utils.params_utils import get_params
from src.utils.wv_loader import Vocab


def train(params):
    # GPU资源配置
    config_gpu(use_cpu=True)

    # 读取vocab训练
    vocab = Vocab(params["vocab_path"], params["vocab_size"])

    params['vocab_size'] = vocab.count

    # 构建模型
    print("Building the model ...")
    model = Seq2Seq(params, vocab)

    # 获取保存管理者
    checkpoint = tf.train.Checkpoint(Seq2Seq=model)
    checkpoint_manager = tf.train.CheckpointManager(checkpoint, params['checkpoint_dir'], max_to_keep=5)

    # 训练模型
    train_model(model, vocab, params, checkpoint_manager)

import tensorflow as tf

# from src.pgn_tf2.batcher import batcher
from src.seq2seq_tf2.seq2seq_batcher import train_batch_generator
import time
from functools import partial


def train_model(model, vocab, params, checkpoint_manager):
    epochs = params['epochs']

    pad_index = vocab.word2id[vocab.PAD_TOKEN]

    # 获取vocab大小
    params['vocab_size'] = vocab.count

    optimizer = tf.keras.optimizers.Adam(name='Adam', learning_rate=params['learning_rate'])

    train_dataset, val_dataset, train_steps_per_epoch, val_steps_per_epoch = train_batch_generator(
        params['batch_size'], params['max_enc_len'], params['max_dec_len'], params['buffer_size']
    )

    for epoch in range(epochs):
        start = time.time()
        enc_hidden = model.encoder.initialize_hidden_state()

        total_loss = 0.
        running_loss = 0.
        for (batch, (inputs, target)) in enumerate(train_dataset.take(train_steps_per_epoch), start=1):

            batch_loss = train_step(model, inputs, target, enc_hidden,
                                    loss_function=partial(loss_function, pad_index=pad_index),
                                    optimizer=optimizer)
            total_loss += batch_loss

            if batch % 50 == 0:
                print('Epoch {} Batch {} Loss {:.4f}'.format(epoch + 1,
                                                             batch,
                                                             (total_loss - running_loss) / 50))
                running_loss = total_loss
        # saving (checkpoint) the model every 2 epochs
        if (epoch + 1) % 2 == 0:
            ckpt_save_path = checkpoint_manager.save()
            print('Saving checkpoint for epoch {} at {}'.format(epoch + 1,
                                                                ckpt_save_path))

        valid_loss = evaluate(model, val_dataset, val_steps_per_epoch,
                              loss_func=partial(loss_function, pad_index=pad_index))

        print('Epoch {} Loss {:.4f}; val Loss {:.4f}'.format(
            epoch + 1, total_loss / train_steps_per_epoch, valid_loss)
        )

        print('Time taken for 1 epoch {} sec\n'.format(time.time() - start))


# 定义损失函数
def loss_function(real, pred, pad_index):
    loss_object = tf.keras.losses.SparseCategoricalCrossentropy(from_logits=True, reduction='none')
    mask = tf.math.logical_not(tf.math.equal(real, pad_index))
    loss_ = loss_object(real, pred)
    mask = tf.cast(mask, dtype=loss_.dtype)
    loss_ *= mask
    return tf.reduce_mean(loss_)


def train_step(model, enc_inp, dec_target, enc_hidden, loss_function=None, optimizer=None, mode='train'):
    with tf.GradientTape() as tape:

        enc_output, enc_hidden = model.encoder(enc_inp, enc_hidden)
        # 第一个隐藏层输入
        dec_hidden = enc_hidden

        # 逐个预测序列
        predictions, _ = model.teacher_decoder(dec_hidden, enc_output, dec_target)

        batch_loss = loss_function(dec_target[:, 1:], predictions)

        if mode == 'train':
            variables = (model.encoder.trainable_variables + model.decoder.trainable_variables
                         + model.attention.trainable_variables)

            gradients = tape.gradient(batch_loss, variables)

            gradients, _ = tf.clip_by_global_norm(gradients, 1.0)

            optimizer.apply_gradients(zip(gradients, variables))

        return batch_loss


def evaluate(model, val_dataset, val_steps_per_epoch, loss_func):
    print('Starting evaluate ...')
    total_loss = 0.
    enc_hidden = model.encoder.initialize_hidden_state()
    for (batch, (inputs, target)) in enumerate(val_dataset.take(val_steps_per_epoch), start=1):
        batch_loss = train_step(model, inputs, target, enc_hidden,
                                loss_function=loss_func, mode='val')
        total_loss += batch_loss
    return total_loss / val_steps_per_epoch

from src.build_data.data_loader import load_dataset 
import tensorflow as tf
from src.utils import config
from tqdm import tqdm


def train_batch_generator(batch_size, max_enc_len=200, max_dec_len=50, buffer_size=5, sample_sum=None):
    # 加载数据集
    train_X, train_Y = load_dataset(config.train_x_path, config.train_y_path,
                                    max_enc_len, max_dec_len)
    val_X, val_Y = load_dataset(config.test_x_path, config.test_y_path,
                                max_enc_len, max_dec_len)
    if sample_sum:
        train_X = train_X[:sample_sum]
        train_Y = train_Y[:sample_sum]
    print(f'total {len(train_Y)} examples ...')
    train_dataset = tf.data.Dataset.from_tensor_slices((train_X, train_Y)).shuffle(len(train_X),
                                                                                   reshuffle_each_iteration=True)
    val_dataset = tf.data.Dataset.from_tensor_slices((val_X, val_Y)).shuffle(len(val_X),
                                                                             reshuffle_each_iteration=True)
    train_dataset = train_dataset.batch(batch_size, drop_remainder=True).prefetch(buffer_size)
    val_dataset = val_dataset.batch(batch_size, drop_remainder=True).prefetch(buffer_size)
    train_steps_per_epoch = len(train_X) // batch_size
    val_steps_per_epoch = len(val_X) // batch_size
    return train_dataset, val_dataset, train_steps_per_epoch, val_steps_per_epoch

def load_dataset(x_path, y_path, max_enc_len, max_dec_len, sample_sum=None):
    x = np.load(x_path+".npy")
    y = np.load(y_path+".npy")

    if sample_sum:
        x = x[:sample_sum, :max_enc_len]
        y = y[:sample_sum, :max_dec_len]
    else:
        x = x[:, :max_enc_len]
        y = y[:, :max_dec_len]
    return x, y