原理介绍

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更多资料：
https://distill.pub/2016/augmented-rnns/#attentional-interfaces
https://www.cnblogs.com/shixiangwan/p/7573589.html#top
http://baijiahao.baidu.com/s?id=1587926245504773589&wfr=spider&for=pc
https://talbaumel.github.io/blog/attention/
浅谈Attention-based Model【原理篇】

论文阅读

Hierarchical Attention Networks for Document Classification
这篇文章主要讲述了基于Attention机制实现文本分类

假设我们有很多新闻文档，这些文档属于三类：军事、体育、娱乐。其中有一个文档D有L个句子s_i（i代表s是文档D的第i个句子），每个句子包含T_i个词（word），w_it代表第i个句子的word,t∈[0,T]

Word Encoder：
①给定一个句子s_i,例如 The superstar is walking in the street，由下面表示[w_i1,w_i2,w_i3,w_i4,w_i5,w_i6,w_i1,w_i7]，我们使用一个词嵌入矩阵W将单词编码为向量

②使用双向GRU编码整个句子关于单词w_it的隐含向量：

那么最终隐含向量为前向隐含向量和后向隐含向量拼接在一起

Word Attention:
给定一句话，并不是这个句子中所有的单词对个句子语义起同等大小的“贡献”，比如上句话“The”,“is”等，这些词没有太大作用，因此我们需要使用attention机制来提炼那些比较重要的单词，通过赋予权重以提高他们的重要性。
①通过一个MLP获取h_it的隐含表示：

②通过一个softmax函数获取归一化的权重：

③计算句子向量：
通过每个单词获取的h_it与对应权重α_it乘积，然后获取获得句子向量

代码实现

attenton.py

import tensorflow as tf


def attention(inputs, attention_size, time_major=False, return_alphas=False):


    if isinstance(inputs, tuple):
        # In case of Bi-RNN, concatenate the forward and the backward RNN outputs.
        inputs = tf.concat(inputs, 2)

    if time_major:
        # (T,B,D) => (B,T,D)
        inputs = tf.array_ops.transpose(inputs, [1, 0, 2])

    hidden_size = inputs.shape[2].value  # D value - hidden size of the RNN layer

    # Trainable parameters
    w_omega = tf.Variable(tf.random_normal([hidden_size, attention_size], stddev=0.1))
    b_omega = tf.Variable(tf.random_normal([attention_size], stddev=0.1))
    u_omega = tf.Variable(tf.random_normal([attention_size], stddev=0.1))

    with tf.name_scope('v'):
        # Applying fully connected layer with non-linear activation to each of the B*T timestamps;
        #  the shape of `v` is (B,T,D)*(D,A)=(B,T,A), where A=attention_size
        v = tf.tanh(tf.tensordot(inputs, w_omega, axes=1) + b_omega)

    # For each of the timestamps its vector of size A from `v` is reduced with `u` vector
    vu = tf.tensordot(v, u_omega, axes=1, name='vu')  # (B,T) shape
    alphas = tf.nn.softmax(vu, name='alphas')         # (B,T) shape

    # Output of (Bi-)RNN is reduced with attention vector; the result has (B,D) shape
    output = tf.reduce_sum(inputs * tf.expand_dims(alphas, -1), 1)

    if not return_alphas:
        return output
    else:
        return output, alphas

train.py

from __future__ import print_function, division

import numpy as np
import tensorflow as tf
from keras.datasets import imdb
from tensorflow.contrib.rnn import GRUCell
from tensorflow.python.ops.rnn import bidirectional_dynamic_rnn as bi_rnn
from tqdm import tqdm

from attention import attention
from utils import get_vocabulary_size, fit_in_vocabulary, zero_pad, batch_generator

NUM_WORDS = 10000
INDEX_FROM = 3
SEQUENCE_LENGTH = 250
EMBEDDING_DIM = 100
HIDDEN_SIZE = 150
ATTENTION_SIZE = 50
KEEP_PROB = 0.8
BATCH_SIZE = 256
NUM_EPOCHS = 3  # Model easily overfits without pre-trained words embeddings, that's why train for a few epochs
DELTA = 0.5
MODEL_PATH = './model'

# Load the data set
(X_train, y_train), (X_test, y_test) = imdb.load_data(num_words=NUM_WORDS, index_from=INDEX_FROM)

# Sequences pre-processing
vocabulary_size = get_vocabulary_size(X_train)
X_test = fit_in_vocabulary(X_test, vocabulary_size)
X_train = zero_pad(X_train, SEQUENCE_LENGTH)
X_test = zero_pad(X_test, SEQUENCE_LENGTH)

# Different placeholders
with tf.name_scope('Inputs'):
    batch_ph = tf.placeholder(tf.int32, [None, SEQUENCE_LENGTH], name='batch_ph')
    target_ph = tf.placeholder(tf.float32, [None], name='target_ph')
    seq_len_ph = tf.placeholder(tf.int32, [None], name='seq_len_ph')
    keep_prob_ph = tf.placeholder(tf.float32, name='keep_prob_ph')

# Embedding layer
with tf.name_scope('Embedding_layer'):
    embeddings_var = tf.Variable(tf.random_uniform([vocabulary_size, EMBEDDING_DIM], -1.0, 1.0), trainable=True)
    tf.summary.histogram('embeddings_var', embeddings_var)
    batch_embedded = tf.nn.embedding_lookup(embeddings_var, batch_ph)

# (Bi-)RNN layer(-s)
rnn_outputs, _ = bi_rnn(GRUCell(HIDDEN_SIZE), GRUCell(HIDDEN_SIZE),
                        inputs=batch_embedded, sequence_length=seq_len_ph, dtype=tf.float32)
tf.summary.histogram('RNN_outputs', rnn_outputs)

# Attention layer
with tf.name_scope('Attention_layer'):
    attention_output, alphas = attention(rnn_outputs, ATTENTION_SIZE, return_alphas=True)
    tf.summary.histogram('alphas', alphas)

# Dropout
drop = tf.nn.dropout(attention_output, keep_prob_ph)

# Fully connected layer
with tf.name_scope('Fully_connected_layer'):
    W = tf.Variable(tf.truncated_normal([HIDDEN_SIZE * 2, 1], stddev=0.1))  # Hidden size is multiplied by 2 for Bi-RNN
    b = tf.Variable(tf.constant(0., shape=[1]))
    y_hat = tf.nn.xw_plus_b(drop, W, b)
    y_hat = tf.squeeze(y_hat)
    tf.summary.histogram('W', W)

with tf.name_scope('Metrics'):
    # Cross-entropy loss and optimizer initialization
    loss = tf.reduce_mean(tf.nn.sigmoid_cross_entropy_with_logits(logits=y_hat, labels=target_ph))
    tf.summary.scalar('loss', loss)
    optimizer = tf.train.AdamOptimizer(learning_rate=1e-3).minimize(loss)

    # Accuracy metric
    accuracy = tf.reduce_mean(tf.cast(tf.equal(tf.round(tf.sigmoid(y_hat)), target_ph), tf.float32))
    tf.summary.scalar('accuracy', accuracy)

merged = tf.summary.merge_all()

# Batch generators
train_batch_generator = batch_generator(X_train, y_train, BATCH_SIZE)
test_batch_generator = batch_generator(X_test, y_test, BATCH_SIZE)

train_writer = tf.summary.FileWriter('./logdir/train', accuracy.graph)
test_writer = tf.summary.FileWriter('./logdir/test', accuracy.graph)

session_conf = tf.ConfigProto(gpu_options=tf.GPUOptions(allow_growth=True))

saver = tf.train.Saver()

if __name__ == "__main__":
    with tf.Session(config=session_conf) as sess:
        sess.run(tf.global_variables_initializer())
        print("Start learning...")
        for epoch in range(NUM_EPOCHS):
            loss_train = 0
            loss_test = 0
            accuracy_train = 0
            accuracy_test = 0

            print("epoch: {}\t".format(epoch), end="")

            # Training
            num_batches = X_train.shape[0] // BATCH_SIZE
            for b in tqdm(range(num_batches)):
                x_batch, y_batch = next(train_batch_generator)
                seq_len = np.array([list(x).index(0) + 1 for x in x_batch])  # actual lengths of sequences
                loss_tr, acc, _, summary = sess.run([loss, accuracy, optimizer, merged],
                                                    feed_dict={batch_ph: x_batch,
                                                               target_ph: y_batch,
                                                               seq_len_ph: seq_len,
                                                               keep_prob_ph: KEEP_PROB})
                accuracy_train += acc
                loss_train = loss_tr * DELTA + loss_train * (1 - DELTA)
                train_writer.add_summary(summary, b + num_batches * epoch)
            accuracy_train /= num_batches

            # Testing
            num_batches = X_test.shape[0] // BATCH_SIZE
            for b in tqdm(range(num_batches)):
                x_batch, y_batch = next(test_batch_generator)
                seq_len = np.array([list(x).index(0) + 1 for x in x_batch])  # actual lengths of sequences
                loss_test_batch, acc, summary = sess.run([loss, accuracy, merged],
                                                         feed_dict={batch_ph: x_batch,
                                                                    target_ph: y_batch,
                                                                    seq_len_ph: seq_len,
                                                                    keep_prob_ph: 1.0})
                accuracy_test += acc
                loss_test += loss_test_batch
                test_writer.add_summary(summary, b + num_batches * epoch)
            accuracy_test /= num_batches
            loss_test /= num_batches

            print("loss: {:.3f}, val_loss: {:.3f}, acc: {:.3f}, val_acc: {:.3f}".format(
                loss_train, loss_test, accuracy_train, accuracy_test
            ))
        train_writer.close()
        test_writer.close()
        saver.save(sess, MODEL_PATH)
        print("Run 'tensorboard --logdir=./logdir' to checkout tensorboard logs.")

utils.py

from __future__ import print_function

import numpy as np


def zero_pad(X, seq_len):
    return np.array([x[:seq_len - 1] + [0] * max(seq_len - len(x), 1) for x in X])


def get_vocabulary_size(X):
    return max([max(x) for x in X]) + 1  # plus the 0th word


def fit_in_vocabulary(X, voc_size):
    return [[w for w in x if w < voc_size] for x in X]


def batch_generator(X, y, batch_size):
    """Primitive batch generator 
    """
    size = X.shape[0]
    X_copy = X.copy()
    y_copy = y.copy()
    indices = np.arange(size)
    np.random.shuffle(indices)
    X_copy = X_copy[indices]
    y_copy = y_copy[indices]
    i = 0
    while True:
        if i + batch_size <= size:
            yield X_copy[i:i + batch_size], y_copy[i:i + batch_size]
            i += batch_size
        else:
            i = 0
            indices = np.arange(size)
            np.random.shuffle(indices)
            X_copy = X_copy[indices]
            y_copy = y_copy[indices]
            continue


if __name__ == "__main__":
    # Test batch generator
    gen = batch_generator(np.array(['a', 'b', 'c', 'd']), np.array([1, 2, 3, 4]), 2)
    for _ in range(8):
        xx, yy = next(gen)
        print(xx, yy)

代码地址：https://github.com/ilivans/tf-rnn-attention

运行结果：

在训练集上准确率达到96%，测试集达到86%，效果还是很强大