python - Tensorflow 循环神经网络 : how to infer a sequence without duplicates?

Question

我正在使用 seq2seq RNN 生成给定种子标签的标签输出序列。在推理步骤中，我想生成仅包含唯一标签的序列(即跳过已经添加到输出序列的标签)。为此，我创建了一个采样器对象，它试图记住已添加到输出中的标签并将它们的 logit 值减少到 -np.inf。

这是采样器代码:

class InferenceSampler(object):
    def __init__(self, out_weights, out_biases):
        self._out_weights = tf.transpose(out_weights)
        self._out_biases = out_biases

        self._n_tracks = out_weights.shape[0]
        self.ids_mask = tf.zeros([self._n_tracks], name="playlist_mask")

    def __call__(self, decoder_outputs):
        _logits = tf.matmul(decoder_outputs, self._out_weights)
        _logits = tf.nn.bias_add(_logits, self._out_biases)

        # apply mask
        _logits = _logits + self.ids_mask

        _sample_ids = tf.cast(tf.argmax(_logits, axis=-1), tf.int32)

        # update mask
        step_ids_mask = tf.sparse_to_dense(_sample_ids, [self._n_tracks], -np.inf)
        self.ids_mask = self.ids_mask + step_ids_mask

        return _sample_ids

推理图的代码如下所示:

self._max_playlist_len = tf.placeholder(tf.int32, ())
self._start_tokens = tf.placeholder(tf.int32, [None])

sample_fn = InferenceSampler(out_weights, out_biases)
with tf.name_scope("inf_decoder"):
    def _end_fn(sample_ids):
        return tf.equal(sample_ids, PAD_ITEM_ID)

    def _next_inputs_fn(sample_ids):
        return tf.nn.embedding_lookup(
            track_embs,
            sample_ids
        )

    _start_inputs = tf.nn.embedding_lookup(
        track_embs,
        self._start_tokens
    )

    helper = tf.contrib.seq2seq.InferenceHelper(
        sample_fn=sample_fn,
        sample_shape=[],
        sample_dtype=tf.int32,
        start_inputs=_start_inputs,
        end_fn=_end_fn,
        next_inputs_fn=_next_inputs_fn
    )
    decoder = tf.contrib.seq2seq.BasicDecoder(
        rnn_cell,
        helper,
        rnn_cell.zero_state(tf.shape(self._start_tokens)[0], tf.float32),
        output_layer=projection_layer
    )
    outputs, _, _ = tf.contrib.seq2seq.dynamic_decode(
        decoder,
        maximum_iterations=self._max_playlist_len
    )

self.playlists = outputs.sample_id

不幸的是，结果仍然有重复的标签。此外，当我尝试访问 sample_fn.ids_mask 时，我收到一条错误消息:ValueError: Operation 'inf_decoder/decoder/while/BasicDecoderStep/add_1' has been marked as not fetchable。

我做错了什么？创建这样的 sample_fn 是否合法？

Answer 1

为了克服这个问题，我更新了推理，在每个 RNN 步骤中输出嵌入向量而不是 item_id。推理完成后，我将嵌入转换为 item_ids。

首先，该解决方案最大限度地减少了操作次数。其次，由于我使用 LSTM/GRU 单元，它们最大限度地降低了在 RNN 推理的不同步骤中观察到两个绝对相似输出的概率。

新代码如下所示:

with tf.name_scope("inf_decoder"):
    def _sample_fn(decoder_outputs):
        return decoder_outputs

    def _end_fn(sample_ids):
        # infinite
        return tf.tile([False], [n_seeds])

    _start_inputs = tf.nn.embedding_lookup(
        track_embs,
        self._seed_items
    )

    helper = tf.contrib.seq2seq.InferenceHelper(
        sample_fn=_sample_fn,
        sample_shape=[self.emb_size],
        sample_dtype=tf.float32,
        start_inputs=_start_inputs,
        end_fn=_end_fn,
    )
    decoder = tf.contrib.seq2seq.BasicDecoder(
        rnn_cell,
        helper,
        rnn_cell.zero_state(n_seeds, tf.float32),
        output_layer=projection_layer
    )
    outputs, _, _ = tf.contrib.seq2seq.dynamic_decode(
        decoder,
        maximum_iterations=self._max_playlist_len
    )

flat_rnn_output = tf.reshape(outputs.rnn_output, [-1, self.emb_size])
flat_logits = tf.matmul(flat_rnn_output, out_weights, transpose_b=True)
flat_logits = tf.nn.bias_add(flat_logits, out_biases)

item_ids = tf.cast(tf.argmax(flat_logits, axis=-1), tf.int32)
playlists = tf.reshape(item_ids, [n_seeds, -1])

self.playlists = playlists