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无所不能的Embedding4 – Doc2vec第二弹[skip-thought & tf-Seq2Seq源码解析]

开发技术 开发技术 5天前 11次浏览

前一章Doc2Vec里提到,其实Doc2Vec只是通过加入Doc_id捕捉了文本的主题信息,并没有真正考虑语序以及上下文语义,n-gram只能在局部解决这一问题,那么还有别的解决方案么?依旧是通用文本向量,skip-thought尝试应用encoder-decoder来学习包含上下文信息和语序的句子向量。魔改后的实现可以看这里( ´▽`) github-DSXiangLi-Embedding-skip_thought

Skip-Thought模型分析

无所不能的Embedding4 - Doc2vec第二弹[skip-thought & tf-Seq2Seq源码解析]

Skip-Thought顾名思义是沿用了skip-gram的路子,不熟悉的童鞋看这里 无所不能的Embedding1 – Word2vec模型详解&代码实现

skip-gram是用中间词来预测周围单词,skip-Thought是用中间句子来预测前一个句子和后一个句子,模型思路就是这么简单粗暴,具体实现就涉及到句子的信息要如何提取,以及loss function的选择。作者选择了encoder-decoder来提取句子信息,用翻译模型常用的log-perplrexity作为loss。

这里想提一句不同模型,在不同的样本上,训练出的文本向量所包含的信息是不同的。例如word2vec的假设就是context(windo_size内周围词)相似的单词更相似(向量空间距离更近)。skip-thought作者对于文本向量的假设是:能更好reconstruct前后句子的信息,就是当前句子的所含信息,换言之前后句子相似的句子,文本向量的空间距离更近。

第一次读到这里感觉哇make perfect sense!可越琢磨越觉着这个task有些迷幻,word2vec skip-gram可以这么搞,是因为给定中间词window_size内的单词选择是相对有限的。你给我个句子就让我精准预测前后句子的每一个词,这能收敛?you what?! 不着急后面似乎有反转~

Encoder部分负责提取中间句子的信息生成定长向量output_state,Decoder则基于ouput_state进行迭代生成前(后)句子。Encoder-Decoder支持任意记忆单元,这里作者选择了GRU-GRU。

简单回顾下GRU Cell,GRU有两个Gate,从两个角度衡量历史sequence信息和当前token的相关程度,(Gamma_r)控制多少历史信息参与state的重新计算是reset gate,(Gamma_u)控制多少历史信息直接进入当前state是update gate,这里安利一篇博客 Illustrated Guide to LSTM’s and GRU’s: A step by step explanation

[begin{align}
Gamma_u & =sigma(w_{u}[h^{<t-1>},x^{<t>}]+ b_{u})\
Gamma_r & =sigma(w_{r}[h^{<t-1>},x^{<t>}]+ b_{r})\
h^{<t>} &= tanh( w_{a}[ Gamma_r odot h^{<t-1>},x^{<t>}]+ b_{a} )\
h^{<t>} &= Gamma_u odot h^{<t>} + (1-Gamma_u) odot
h^{<t-1>}
end{align}
]

Encoder部分经过GRU把长度为T的sequence信息压缩到hidden_size的(h^{<T>}),这里(h^{<T>})也是最终skip-thought为每一个句子生成的通用向量表达。

Decoder部分基于(h^{<T>})向前预测下一个/上一个句子中的每一个单词。Decoder比Encoder略复杂,在于训练阶段和预测阶段对于input的处理存在差异。

训练阶段使用了100%的Teacher Forcing,每个cell的输入除了上一个cell的hidden state,还有预测句子中前一个真实token对应的embedding,如图

无所不能的Embedding4 - Doc2vec第二弹[skip-thought & tf-Seq2Seq源码解析]

而在预测阶段真实序列未知,因此会转而使用前一个cell的output来预测前一个token,再用预测token的embedding作为输入,如图

无所不能的Embedding4 - Doc2vec第二弹[skip-thought & tf-Seq2Seq源码解析]

对于翻译模型来说,在训练阶段使用TeacherForcing的好处是可以加速模型收敛,避免向前迭代预测的误差进一步放大。坏处自然是训练和预测时decoder的表现存在差异(Exposure Bias),以及预测时decode的output会受到训练样本的约束。这里最常用的解决方案是Scheduled Sampling, 简单来说就是在训练阶段有P的概率输入用teacher forcing,1-P的概率用预测output。但是!skip-thought并没有使用这个解决方案,为啥嘞?反转来了V(^_^)V

看到无采样的teacherforcing这里,前面的迷惑已然解答。其实skip-thought并不只是使用中间句子来预测前后句子,而是基于中间句子的ouput_state,用前后句子中T-1前的单词来预测第T个单词(感觉和missing imputation只有一步之遥)。encoder部分只需要在output_state中最大程度的提取句子信息,保证在不同的前后句子上output state都可以generalize。至于decoder的预测部分效果如何模型并不关心,因为skip-thought的预测输出就是encoder部分的output state,所以自然是不需要使用Scheduled Sampling

skip-thought的Decoder还有两点特殊:

  • 前/后句子用两个decoder来训练,两个decoder除了word embedding共享之外,参数独立
  • encoder state不只作为decoder的initial_state,而是直接传入decoder的每一个cell,起到类似residual/connditional的作用,避免encoder的信息传着传着给传没了。这里感觉不用attention而是直接传入outpu_state也是为了保证这个output_state能最大程度的学到用来reconstruct前后句子的信息

loss部分作者用了语言模型的log-perplexity把前后句子的loss加总得到loss function

[sum_t logP(w_{i+1}^t| w_{i+1}^{<t}, h_i) + logP(w_{i-1}^t| w_{i-1}^{<t}, h_i)
]

论文比较有意思的一个点还有vocabulary expansion,就是如何把word embedding扩展到训练集之外。作者尝试用linear-mapping的方式学习word2vec和skip-thought里面word-embedding的映射关系,就是找到word2vec和skip-thought交集的word, 对他们的embedding做regression $ X_{word2vec} sim W cdot X_{skipthought} $,这样对样本外但是word2vec内的单词直接用W映射就能得到skip-thougt的词向量

这里直接用word2vec/glove的word embedding来初始化skip-thougt的词向量是不是更好?在后面的模型实现里我就是直接用word2vec来初始化了embedding, word2vec之外词用random.uniform(-0.1,0.1)来初始化

最终在生成文本向量的时候,作者给出了几种方案,遵循大力一定出奇迹的原则自然方案3效果更好

  1. 2400-dim的uni-skip, 就是以上encoder生成的output state
  2. 两个1200-dim的bi-skip向量拼接得到2400-dim的向量,是在处理训练样本时一个用正常语序训练,一个reverse语序训练
  3. 把上面两个2400-dim拼接得到4800-dim

skip-thought 模型实现

这里有点任性的对论文做了魔改。。。部分细节和论文已经天差地别,可以拿来了解encoder-decoder的实现但不保证完全reproduce skip-thought的结果。。。以下只保留代码核心部分,完整代码在 github-DSXiangLi-Embedding-skip_thought。 这里用了tensorflow seq2seq的框架,不熟悉的童鞋可以先看后面seq2seq的代码解析~

dataset

论文中是((s_{i-1}, s_i, s_{i+1}))作为一组样本,其中(s_i)是encoder source,(s_{i-1})(s_{i+1})是decoder target,这里我直接处理成((s_i,s_{i-1})),((s_i,s_{i+1}))两组样本。

其中encoder source不需要多做处理,但是decoder source在Train和Eval时需要在sequence前后加入start和end_token标记序列的开始和结束,在Predict时需要加入start_token标记开始。最后通过word_table把token映射到token_id,再Padding到相同长度就齐活。

这里在Dataset的部分加入了获取word2vec embedding的部分, word2vec以外的单词默认random.uniform(-0.1,0.1)

class SkipThoughtDataset(BaseDataset):
    def __init__(self, data_file, dict_file, epochs, batch_size, buffer_size, min_count, max_count,
                 special_token, max_len):
    ...
    def parse_example(self, line, prepend, append):
        features = {}
        tokens = tf.string_split([tf.string_strip(line)]).values

        if prepend:
            tokens = tf.concat([[self.special_token.SEQ_START], tokens], 0)
        if append:
            tokens = tf.concat([tokens, [self.special_token.SEQ_END]], 0)

        features['tokens'] = tokens
        features['seq_len'] = tf.size(tokens)
        return features
    ...

    def make_source_dataset(self, file_path, data_type, is_predict, word_table_func):
        prepend, append = self.prepend_append_logic(data_type, is_predict)

        dataset = tf.data.TextLineDataset(file_path).
            map(lambda x: self.parse_example(x, prepend, append), num_parallel_calls=tf.data.experimental.AUTOTUNE).
            map(lambda x: word_table_func(x), num_parallel_calls=tf.data.experimental.AUTOTUNE)

        return dataset

    def build_dataset(self, is_predict=0):
        def input_fn():
            word_table_func = self.word_table_lookup(self.build_wordtable())
            _ = self.build_tokentable() # initialize here to ensure lookup table is in the same graph

            encoder_source = self.make_source_dataset(self.data_file['encoder'], 'encoder', is_predict, word_table_func)
            decoder_source = self.make_source_dataset(self.data_file['decoder'], 'decoder', is_predict, word_table_func)

            dataset = tf.data.Dataset.zip((encoder_source, decoder_source)).
                filter(self.sample_filter_logic)

            if not is_predict:
                dataset = dataset.
                    repeat(self.epochs)

                dataset = dataset. 
                    padded_batch( batch_size=self.batch_size,
                                  padded_shapes=self.padded_shape,
                                  padding_values=self.padding_values,
                                  drop_remainder=True ). 
                    prefetch( tf.data.experimental.AUTOTUNE )
            else:
                dataset = dataset.batch(1)

            return dataset
        return input_fn

    def load_pretrain_embedding(self):
        if self.embedding is None:
            word_vector = gensim.downloader.load(PretrainModel)
            embedding = []
            for i in self._dictionary.keys():
                try:
                    embedding.append( word_vector.get_vector( i ) )
                except KeyError:
                    embedding.append( np.random.uniform(low=-0.1, high=0.1, size=300))
            self.embedding = np.array(embedding, dtype=np.float32)
        return self.embedding

Encoder-Decoder

Encoder的部分很常规,确认cell类型,然后经过dynamic_rnn迭代,输出output和state

def gru_encoder(input_emb, input_len, params):
    gru_cell = build_rnn_cell('gru', params)

    # state: batch_size * hidden_size, output: batch_size * max_len * hidden_size
    output, state = tf.nn.dynamic_rnn(
        cell=gru_cell, # one rnn units
        inputs=input_emb, # batch_size * max_len * feature_size
        sequence_length=input_len, # batch_size * seq_len
        initial_state=None,
        dtype=params['dtype'],
        time_major=False # whether reshape max_length to first dim
    )
    return ENCODER_OUTPUT(output=output, state=state)

Decoder的部分可以分成helper, decoder, 以及最终dynamic_decode的部分。比较容易踩坑的有几个点

  • dynamic_decode部分的max_iteration在训练时可以不设定,train_helper会基于seq_len判断finish,在Eval时因为用GreedyEmbeddingHelper所以需要手动传入pad_len,在predict部分只需给定最大max_iter保证预测一定会停止就好
  • Decoder的output layer必须要有, 因为需要做hidden_size -> vocab_size(softmax)的转换,用于预测每个cell的token输出
  • 前面dataset的decoder_source我们在前后都加了开始/停止token,训练时需要移除最后一个token(对于trainHelper可以改inputs也可以改sequence_length), 这样在计算loss时可以和移除第一个token的target对齐。

这里针对上面提到的把encoder的output_state直接传入每个decoder cell做了实现,直接把encoder state和embedding input做了拼接作为输入。

def get_helper(encoder_output, input_emb, input_len, batch_size, embedding, mode, params):

    if mode == tf.estimator.ModeKeys.TRAIN:
        if params['conditional']:
            # conditional train helper with encoder output state as direct input
            # Reshape encoder state as auxiliary input: 1* batch_size * hidden -> batch_size * max_len * hidden
            decoder_length = tf.shape(input_emb)[1]
            state_shape = tf.shape(encoder_output.state)
            encoder_state = tf.tile(tf.reshape(encoder_output.state, [state_shape[1],
                                                                      state_shape[0],
                                                                      state_shape[2]]),
                                    [1, decoder_length, 1])
            input_emb = tf.concat([encoder_state, input_emb], axis=-1)

            helper = seq2seq.TrainingHelper( inputs=input_emb, # batch_size * max_len-1 * emb_size
                                             sequence_length=input_len-1, # exclude last token
                                             time_major=False,
                                             name='training_helper' )
    else:
        helper = seq2seq.GreedyEmbeddingHelper( embedding=embedding_func( embedding ),
                                                start_tokens=tf.fill([batch_size], params['start_token']),
                                                end_token=params['end_token'] )

    return helper
    
def get_decoder(decoder_cell, encoder_output, input_emb, input_len, embedding, output_layer, mode, params):
    batch_size = tf.shape(encoder_output.output)[0]
    if params['beam_width'] >1 :
        # If beam search multiple prediction are uesd at each time step
        decoder = seq2seq.BeamSearchDecoder( cell=decoder_cell,
                                             embedding=embedding_func( embedding ),
                                             initial_state=encoder_output,
                                             beam_width=params['beam_width'],
                                             start_tokens=tf.fill([batch_size], params['start_token']),
                                             end_token=params['end_token'],
                                             output_layer=output_layer )

    else:
        helper = get_helper(encoder_output, input_emb, input_len, batch_size, embedding, mode, params)

        decoder = seq2seq.BasicDecoder( cell=decoder_cell,
                                        helper=helper,
                                        initial_state=encoder_output.state,
                                        output_layer=output_layer )

    return decoder

def gru_decoder(encoder_output, input_emb, input_len, embedding, params, mode):
    gru_cell = build_rnn_cell( 'gru', params )

    if mode == tf.estimator.ModeKeys.TRAIN:
        max_iteration = None
    elif mode == tf.estimator.ModeKeys.EVAL:
        max_iteration = tf.reduce_max(input_len) # decode max sequence length(=padded_length)in EVAL
    else:
        max_iteration = params['max_decode_iter']  # decode pre-defined max_decode iter in predict

    output_layer=tf.layers.Dense(units=params['vocab_size'])  # used for infer helper sample or train loss calculation
    decoder = get_decoder(gru_cell, encoder_output, input_emb, input_len, embedding, output_layer, mode, params)

    output, state, seq_len = seq2seq.dynamic_decode(decoder=decoder,
                                                    output_time_major=False,
                                                    impute_finished=True,
                                                    maximum_iterations=max_iteration)

    return DECODER_OUTPUT(output=output, state = state, seq_len=seq_len)

loss

loss这了自己实现的一版sequence_loss,把计算loss和按不同维度聚合拆成了两块。感觉tf.sequence_loss只针对train,对eval的部分并不友好,因为trainHelper可以保证source和target的长度一致,但是infer时调用GreedyEmbeddingHelper是无法保证输出长度的(不知道是不是我哪里理解错了,如果是请大神指正(o^^o)), 所以对eval部分也做了特殊处理。

def sequence_loss(logits, target, mask, mode):
    with tf.variable_scope('Sequence_loss_matrix'):
        n_class = tf.shape(logits)[2]
        decode_len = tf.shape(logits)[1] # used for infer only, max_len is determined by decoder
        logits = tf.reshape(logits, [-1, n_class])

        if mode == tf.estimator.ModeKeys.TRAIN:
            # In train, target
            target = tf.reshape(target[:, 1:], [-1]) # (batch * (padded_len-1)) * 1
        elif mode == tf.estimator.ModeKeys.EVAL:
            # In eval, target has paded_len, logits have decode_len
            target = tf.reshape(target[:, : decode_len], [-1]) # batch * (decode_len) *1
        else:
            raise Exception('sequence loss is only used in train or eval, not in pure prediction')
        loss_mat = tf.nn.sparse_softmax_cross_entropy_with_logits(labels = target, logits = logits)
        loss_mat = tf.multiply(loss_mat, tf.reshape(mask, [-1])) # apply padded mask on output loss
    return loss_mat
    
def agg_sequence_loss(loss_mat, mask,  axis):
    with tf.variable_scope('Loss_{}'.format(axis)):
        if axis == 'scaler':
            loss = tf.reduce_sum(loss_mat)
            n_sample = tf.reduce_sum(mask)
            loss = loss/n_sample
        else:
            loss_mat = tf.reshape(loss_mat, tf.shape(mask)) # (batch_size * max_len) * 1-> batch_size * max_len
            if axis == 'batch':
                loss = tf.reduce_sum(loss_mat, axis=1) # batch
                n_sample = tf.reduce_sum(mask, axis=1) # batch
                loss = tf.math.divide_no_nan(loss, n_sample) # batch
            elif axis == 'time':
                loss = tf.reduce_sum(loss_mat, axis=0) # max_len
                n_sample = tf.reduce_sum(mask, axis=0) # max_len
                loss = tf.math.divide_no_nan(loss, n_sample) # max_len
            else:
                raise Exception('Only scaler/batch/time are supported in axis param')
    return loss

model

encoder, decoder, loss都ready,拼一块就齐活了, 这里embedding我们用了前面加载的word2vec来进行初始化。

class QuickThought(object):
    def __init__(self, params):
        self.params = params
        self.init()

    def init(self):
        with tf.variable_scope('embedding', reuse=tf.AUTO_REUSE):
            self.embedding = tf.get_variable(dtype = self.params['dtype'],
                                             initializer=tf.constant(self.params['pretrain_embedding']),
                                             name='word_embedding' )

            add_layer_summary(self.embedding.name, self.embedding)

    def build_model(self, features, labels, mode):
        encoder_output = self._encode(features)
        decoder_output = self._decode(encoder_output, labels, mode )

        loss_output = self.compute_loss( decoder_output, labels, mode )

        ...
    def _encode(self, features):
        with tf.variable_scope('encoding'):
            encoder = ENCODER_FAMILY[self.params['encoder_type']]

            seq_emb_input = tf.nn.embedding_lookup(self.embedding, features['tokens']) # batch_size * max_len * emb_size

            encoder_output = encoder(seq_emb_input, features['seq_len'], self.params) # batch_size
        return encoder_output

    def _decode(self, encoder_output, labels, mode):
        with tf.variable_scope('decoding'):
            decoder = DECODER_FAMILY[self.params['decoder_type']]

            if mode == tf.estimator.ModeKeys.TRAIN:
                seq_emb_output = tf.nn.embedding_lookup(self.embedding, labels['tokens']) # batch_size * max_len * emb_size
                input_len = labels['seq_len']
            elif mode == tf.estimator.ModeKeys.EVAL:
                seq_emb_output = None
                input_len = labels['seq_len']
            else:
                seq_emb_output = None
                input_len = None

            decoder_output = decoder(encoder_output, seq_emb_output, input_len,
                                     self.embedding, self.params, mode)
        return decoder_output
    def compute_loss(self, decoder_output, labels, mode):
        with tf.variable_scope('compute_loss'):
            mask = sequence_mask(decoder_output, labels, self.params, mode)

            loss_mat = sequence_loss(logits=decoder_output.output.rnn_output,
                                     target=labels['tokens'],
                                     mask=mask,
                                     mode=mode)
            loss = []
            for axis in ['scaler', 'batch', 'time']:
                loss.append(agg_sequence_loss(loss_mat, mask, axis))

        return SEQ_LOSS_OUTPUT(loss_id=loss_mat,
                               loss_scaler=loss[0],
                               loss_per_batch=loss[1],
                               loss_per_time=loss[2])

tf.seq2seq 代码解析

稀里糊涂开始用seq2seq,结果盯着shape mismatch的报错险些看到地老天荒,索性咱老老实实看一遍tf的实现, 以下代码只保留了核心部分,完整的官方代码在这里哟 tf.seq2seq.contrib

Encoding

Encoding部分就是一个dynamic_rnn,先看下输入

  • cell:任意类型记忆单元rnn,gru, lstm
  • inputs:rnn输入,一般是[batch_size, max_len, emb_siz] 也就是padding的序列token,经过embedding映射之后作为输入
  • sequence_length: 真实序列长度(不包含padding),用于判断序列是遍历
  • initial_state: encoder最初state,None则默认是zero_state
  • dtype: output数据类型,建议全局设置统一数据类型,不然会有各种mismatch,不要问我是怎么知道的>.<
  • parallel_iteration:内存换速度,没有上下文依赖的op进行并行计算
  • time_major:如果你的输入数据是[max_len, batch_size,emb_siz]则为True,一般为False在dynamic_rnn内部再做reshape。

dynamic_rnn主函数其实只做了输入/输出数据的处理部分,包括

  • reshape_input:对应上面time_major=False, 把输入数据从[batch_size, max_len, emb_siz]转换为[max_len, batch_size,emb_siz]
  • inital_state: 默认是batch size的zero_state
  • reshape_output: output输出是[max_len, batch_size,hidden_siz]转换为[batch_size, max_len, hidden_size]
def dynamic_rnn(cell, inputs, sequence_length=None, initial_state=None,
                dtype=None, parallel_iterations=None, swap_memory=False,
                time_major=False, scope=None):
    flat_input = nest.flatten(inputs)

    if not time_major:
      flat_input = [ops.convert_to_tensor(input_) for input_ in flat_input]
      flat_input = tuple(_transpose_batch_time(input_) for input_ in flat_input)

    batch_size = _best_effort_input_batch_size(flat_input)
    state = cell.zero_state(batch_size, dtype)

    inputs = nest.pack_sequence_as(structure=inputs, flat_sequence=flat_input)

    (outputs, final_state) = _dynamic_rnn_loop(
        cell,
        inputs,
        state,
        parallel_iterations=parallel_iterations,
        swap_memory=swap_memory,
        sequence_length=sequence_length,
        dtype=dtype)

    if not time_major:
      # (T,B,D) => (B,T,D)
      outputs = nest.map_structure(_transpose_batch_time, outputs)
    return (outputs, final_state)

核心计算部分都在_dynamic_rnn_loop,是一个while_loop, 所以需要定义三要素[loop_var, body, condition]

  • loop_var:(time, output_ta, state)
    • time:遍历到第几个token
    • output_ta: 每个cell的输出,padding部分是zero-output
    • state: 最后一个cell的输出,对于padding的序列,只计算到最后一个真实token,之后的state是直接copy through

这里output_ta的shape是(batch, max_len, hidden_units), 对于rnn和GRU,state就是最后一个output, 那shape自然是(1, batch, hidden_units), 但LSTM是有两个hidden state的,一个用于向前传递信息一个用于输出,所以这里state的shape会是(2, batch, hidden_units)

  • body

loop的核心计算部分是lambda: cell(input_t, state),也就是相应记忆单元的计算。当sequence_length给定时,_rnn_step的额外操作其实是对已经遍历完的序列直接copy through(zero_output, last_state)

  def _time_step(time, output_ta_t, state):

    input_t = tuple(ta.read(time) for ta in input_ta)
    input_t = nest.pack_sequence_as(structure=inputs, flat_sequence=input_t)
    call_cell = lambda: cell(input_t, state)

    if sequence_length is not None:
      (output, new_state) = _rnn_step(
          time=time,
          sequence_length=sequence_length,
          min_sequence_length=min_sequence_length,
          max_sequence_length=max_sequence_length,
          zero_output=zero_output,
          state=state,
          call_cell=call_cell,
          state_size=state_size,
          skip_conditionals=True)
    else:
      (output, new_state) = call_cell()

    # Pack state if using state tuples
    output = nest.flatten(output)
    output_ta_t = tuple(ta.write(time, out) for ta, out in zip(output_ta_t, output))

    return (time + 1, output_ta_t, new_state)
  • condition

停止loop的条件loop_bound=min(max_sequence_length, max(1,time_steps) , 其中time_step是输入的max_len维度,也就是padding length, max_sequence_length是输入batch的最大真实长度,如果是batch_padding这两个取值应该是一样的

time_steps = input_shape[0]

if sequence_length is not None:
    min_sequence_length = math_ops.reduce_min(sequence_length)
    max_sequence_length = math_ops.reduce_max(sequence_length)
else:
    max_sequence_length = time_steps

loop_bound = math_ops.minimum(time_steps, math_ops.maximum(1, max_sequence_length))
        
  _, output_final_ta, final_state = control_flow_ops.while_loop(
      cond=lambda time, *_: time < loop_bound,
      body=_time_step,
      loop_vars=(time, output_ta, state),
      parallel_iterations=parallel_iterations,
      maximum_iterations=time_steps,
      swap_memory=swap_memory)

Decoding

Decoding主要有三个组件,Decoder,Helper和dynamic_decode。还有比较特殊独立出来的BeamSearch和Attention,这两个后面用到再说

BasicDecoder

BasicDecoder主要接口有2个

  • initialize生成decode阶段的最初input
  • step实现每一步decode的计算,之后被dynamic_decode的while_loop调用

其中initialize拼接了helper的初始化返回再加上initial_state,也就是encoder最后一步的output_state,helper返回的部分我们放在后面说。

def initialize(self, name=None):
    return self._helper.initialize() + (self._initial_state,)

step部分做了如下操作

  1. 输入上一步的output, state计算下一步的output,这是Decoder的核心计算
  2. 如果定义了output_layer,对output做transform,为啥需要output_layer嘞? 这个看到Helper你就明白了
  3. sample, next_inputs: 都是调用Helper的接口
  4. 输出: BasicDecoderOutput(rnn_output, sample_id), next_state, next_inputs, finished
class BasicDecoderOutput(
    collections.namedtuple("BasicDecoderOutput", ("rnn_output", "sample_id"))):
  pass

class BasicDecoder(decoder.Decoder):
  """Basic sampling decoder."""

  def __init__(self, cell, helper, initial_state, output_layer=None):

  def step(self, time, inputs, state, name=None):
    with ops.name_scope(name, "BasicDecoderStep", (time, inputs, state)):
      cell_outputs, cell_state = self._cell(inputs, state)
      if self._output_layer is not None:
        cell_outputs = self._output_layer(cell_outputs)
      sample_ids = self._helper.sample(
          time=time, outputs=cell_outputs, state=cell_state)
      (finished, next_inputs, next_state) = self._helper.next_inputs(
          time=time,
          outputs=cell_outputs,
          state=cell_state,
          sample_ids=sample_ids)
    outputs = BasicDecoderOutput(cell_outputs, sample_ids)
    return (outputs, next_state, next_inputs, finished)

这里发现BasicDecoder的实现只包括了承上的部分,启下的部分都放在了Helper里面,下面我们具体看下Helper的next_input和Sample接口干了啥

Helper

我们主要看两个helper一个用于训练,一个用于预测,主要实现3个接口

  • initialize:生成decode阶段的最初input
  • sample:生成decode下一步的input id
  • next_inputs:生成decode下一步的input

TrainHelper用于训练,sample接口实际并没有用,next_input把sample_id定义为unused_kwargs.

  • initialize返回 (finished, next_inputs)
    • finished: 判断当前batch每个sequence是否已经遍历完, sequence_length是不包含padded的实际sequencec长度
    • 除非batch里所有seq_length的长度都是0,否则直接读取每个sequence的第一个token作为decoder的初始输入

decoder输入sequence会在预处理时加入start_token标记seq的开始,对应上图的(<go>)标记,同时加入start_token也为了形成source和target的错位,做到输入T-1个字符预测T个字符。例如source是[(<go>), I, love, you],target是[I, love, you, (<eos>)]

  • next_inputs输出(finished, next_inputs, state)
    • finished: 判断当前batch每个sequence是否已经遍历完, sequence_length是不包含padded的实际sequencec长度
    • next_inputs: 训练时使用Teaching Force,传入下一个decoder cell的就是前一个位置的实际token embedding,所以这里next_input直接读取input sequence的下一个值,如果finished都是True就返回0【其实返回啥都无所谓因为在loss那里padded的部分会被mask掉】
    • state: 这里是打酱油的,直接pass-throuh
class TrainingHelper(Helper):

  def __init__(self, inputs, sequence_length, time_major=False, name=None):
    ...

  def initialize(self, name=None):
    with ops.name_scope(name, "TrainingHelperInitialize"):
      finished = math_ops.equal(0, self._sequence_length)
      all_finished = math_ops.reduce_all(finished)
      next_inputs = control_flow_ops.cond(
          all_finished, lambda: self._zero_inputs,
          lambda: nest.map_structure(lambda inp: inp.read(0), self._input_tas))
      return (finished, next_inputs)
      
  def next_inputs(self, time, outputs, state, name=None, **unused_kwargs):
    """next_inputs_fn for TrainingHelper."""
    with ops.name_scope(name, "TrainingHelperNextInputs",
                        [time, outputs, state]):
      next_time = time + 1
      finished = (next_time >= self._sequence_length)
      all_finished = math_ops.reduce_all(finished)
      def read_from_ta(inp):
        return inp.read(next_time)
      next_inputs = control_flow_ops.cond(
          all_finished, lambda: self._zero_inputs,
          lambda: nest.map_structure(read_from_ta, self._input_tas))
      return (finished, next_inputs, state)

GreedyHelper用于预测

  • initialize返回 (finished, next_inputs)

    • finished: 都是False,因为infer阶段不用判断input_sequence长度
    • next_inputs: 返回start_token对应的embedding,和训练保持一致
  • sample返回sample_id

负责根据每个decoder cell的output计算出现概率最大的token,作为下一个decoder cell的输入,这里也是上面提到需要output_layer的原因,因为需要hidden_size -> vocab_size的变换,才能进一步计算softmax

  • next_input
    • finished: 如果sequence预测为end_token则该sequence预测完成,判断batch里所有sequence是否预测完成
    • next_inputs: 对sample_id做embedding_lookup作为下一步的输入,如果finished都是True就返回start_token
    • state: 继续打酱油
class GreedyEmbeddingHelper(Helper):
  def __init__(self, embedding, start_tokens, end_token):
    self._start_tokens = ops.convert_to_tensor(
        start_tokens, dtype=dtypes.int32, name="start_tokens")
    self._end_token = ops.convert_to_tensor(
        end_token, dtype=dtypes.int32, name="end_token")
    self._start_inputs = self._embedding_fn(self._start_tokens)
    。。。
  def sample(self, time, outputs, state, name=None):
    sample_ids = math_ops.cast(
        math_ops.argmax(outputs, axis=-1), dtypes.int32)
    return sample_ids

  def initialize(self, name=None):
    finished = array_ops.tile([False], [self._batch_size])
    return (finished, self._start_inputs)
    
  def next_inputs(self, time, outputs, state, sample_ids, name=None):
    finished = math_ops.equal(sample_ids, self._end_token)
    all_finished = math_ops.reduce_all(finished)
    next_inputs = control_flow_ops.cond(
        all_finished,
        lambda: self._start_inputs,
        lambda: self._embedding_fn(sample_ids))
    return (finished, next_inputs, state)

dynamic_decode

承上启下的工具都齐活了,要实现对sequence的预测,只剩下一步就是loop,于是有了dynamic_decode,它其实就干了个while_loop的活,于是还是loop三兄弟[loop_vars, condition, body]

  1. loop_vars=[initial_time, initial_outputs_ta, initial_state, initial_inputs, initial_finished, initial_sequence_lengths]

    • initial_finished, initial_inputs, initial_state是上面decoder的initialize返回
    • initial_time, initial_sequennce=0
    • initial_output_ta是每个elemennt都是batch * decoder.output_size的不定长TensorArray, 这里output_size=(rnn_output_size,sample_id_shape),预测返回1个token的sample_id_shape都是scaler, 有output_layer时rnn_output_size=output_layer_size, default= hidden_size
  2. condition: 判断是否所有finished都为True,都遍历完则停止loop

  3. body: loop的核心计算逻辑

    • step:调用Decoder进行每一步的decode计算

    • finished: 这里finished主要由三个逻辑判断(tracks_own_finished我没用过先忽略了哈哈)其余两个是:

      • helper的next_inputs回传的finished:trainHelper判断输入sequence是否遍历完,GreedyEmbeddingHelper判断预测token是否为end_token。
      • max_iteration:只用于预测,为了避免预测token一直不是end_token导致预测无限循环下去,设置一个最大预测长度,训练时max_iteraion应该为空
    • sequence_length: 记录实际预测sequence长度,没有finished的sequence+1

    • impute_finished: 如果sequence已遍历完, 后面的output补0,后面的state不再计算直接pass through当前state

def body(time, outputs_ta, state, inputs, finished, sequence_lengths):
  (next_outputs, decoder_state, next_inputs,
   decoder_finished) = decoder.step(time, inputs, state)

  if maximum_iterations is not None:
    next_finished = math_ops.logical_or(
        next_finished, time + 1 >= maximum_iterations)
  next_sequence_lengths = array_ops.where(
      math_ops.logical_and(math_ops.logical_not(finished), next_finished),
      array_ops.fill(array_ops.shape(sequence_lengths), time + 1),
      sequence_lengths)

  # Zero out output values past finish
  if impute_finished:
    emit = nest.map_structure(
        lambda out, zero: array_ops.where(finished, zero, out),
        next_outputs,
        zero_outputs)
  else:
    emit = next_outputs

  # Copy through states past finish
  def _maybe_copy_state(new, cur):
    # TensorArrays and scalar states get passed through.
    if isinstance(cur, tensor_array_ops.TensorArray):
      pass_through = True
    else:
      new.set_shape(cur.shape)
      pass_through = (new.shape.ndims == 0)
    return new if pass_through else array_ops.where(finished, cur, new)

  if impute_finished:
    next_state = nest.map_structure(
        _maybe_copy_state, decoder_state, state)
  else:
    next_state = decoder_state

  outputs_ta = nest.map_structure(lambda ta, out: ta.write(time, out),
                                  outputs_ta, emit)
  return (time + 1, outputs_ta, next_state, next_inputs, next_finished,
          next_sequence_lengths)

欢迎留言吐槽以及评论哟~


Reference

skip-thought

  1. Ryan Kiros, yukun Zhu. 2015. SKip-Thought Vectors
  2. Lajanugen logeswaran, Honglak Lee. 2018. An Efficient Framework for Learning Sentence Representations.
  3. https://zhuanlan.zhihu.com/p/64342563
  4. https://towardsdatascience.com/document-embedding-techniques-fed3e7a6a25d
  5. https://towardsdatascience.com/the-best-document-similarity-algorithm-in-2020-a-beginners-guide-a01b9ef8cf05
  6. https://blog.floydhub.com/when-the-best-nlp-model-is-not-the-best-choice/
  7. https://towardsdatascience.com/document-embedding-techniques-fed3e7a6a25d
  8. https://zhuanlan.zhihu.com/p/72575806
  9. Scheduled Sampling for Sequence Prediction with Recurrent Neural Networks

tensorflow seq2seq

  1. https://github.com/google/seq2seq/tree/master/seq2seq
  2. https://zhuanlan.zhihu.com/p/27608348
  3. https://zhuanlan.zhihu.com/p/47929039
  4. https://blog.csdn.net/llh_1178/article/details/89322373

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