Transformer

  def bottom_simple(self, x, name, reuse):
    with tf.variable_scope(name, reuse=reuse):
      # Squeeze out the channels dimension.
      x = tf.squeeze(x, axis=3)
      var = self._get_weights()
      ret = tf.gather(var, x)
      if self._model_hparams.multiply_embedding_mode == "sqrt_depth":
        ret *= self._body_input_depth**0.5
      ret *= tf.expand_dims(tf.to_float(tf.not_equal(x, 0)), -1)
      return ret

  def bottom(self, x):
    if self._model_hparams.shared_embedding_and_softmax_weights:
      return self.bottom_simple(x, "shared", reuse=None)
    else:
      return self.bottom_simple(x, "input_emb", reuse=None)

  def targets_bottom(self, x):
    if self._model_hparams.shared_embedding_and_softmax_weights:
      return self.bottom_simple(x, "shared", reuse=True)
    else:
      return self.bottom_simple(x, "target_emb", reuse=None)

  def _get_weights(self):
    """Create or get concatenated embedding or softmax variable.

    Returns:
       a list of self._num_shards Tensors.
    """
    num_shards = self._model_hparams.symbol_modality_num_shards
    shards = []
    for i in xrange(num_shards):
      shard_size = (self._vocab_size // num_shards) + (
          1 if i < self._vocab_size % num_shards else 0)
      var_name = "weights_%d" % i
      shards.append(
          tf.get_variable(
              var_name, [shard_size, self._body_input_depth],
              initializer=tf.random_normal_initializer(
                  0.0, self._body_input_depth**-0.5)))
    if num_shards == 1:
      ret = shards[0]
    else:
      ret = tf.concat(shards, 0)
    ret = eu.ConvertGradientToTensor(ret)
    return ret

      multiply_embedding_mode="sqrt_depth",
      norm_type="none",  # "batch", layer", "noam", "none".
      layer_norm_epsilon=1e-6,
      symbol_modality_num_shards=16,

def add_timing_signal_1d(x, min_timescale=1.0, max_timescale=1.0e4):
  """Adds a bunch of sinusoids of different frequencies to a Tensor.

  Each channel of the input Tensor is incremented by a sinusoid of a different
  frequency and phase.

  This allows attention to learn to use absolute and relative positions.
  Timing signals should be added to some precursors of both the query and the
  memory inputs to attention.

  The use of relative position is possible because sin(x+y) and cos(x+y) can be
  experessed in terms of y, sin(x) and cos(x).

  In particular, we use a geometric sequence of timescales starting with
  min_timescale and ending with max_timescale.  The number of different
  timescales is equal to channels / 2. For each timescale, we
  generate the two sinusoidal signals sin(timestep/timescale) and
  cos(timestep/timescale).  All of these sinusoids are concatenated in
  the channels dimension.

  Args:
    x: a Tensor with shape [batch, length, channels]
    min_timescale: a float
    max_timescale: a float

  Returns:
    a Tensor the same shape as x.
  """
  length = tf.shape(x)[1]
  channels = tf.shape(x)[2]
  position = tf.to_float(tf.range(length))
  num_timescales = channels // 2
  log_timescale_increment = (
      math.log(float(max_timescale) / float(min_timescale)) /
      (tf.to_float(num_timescales) - 1))
  inv_timescales = min_timescale * tf.exp(
      tf.to_float(tf.range(num_timescales)) * -log_timescale_increment)
  scaled_time = tf.expand_dims(position, 1) * tf.expand_dims(inv_timescales, 0)
  signal = tf.concat([tf.sin(scaled_time), tf.cos(scaled_time)], axis=1)
  signal = tf.pad(signal, [[0, 0], [0, tf.mod(channels, 2)]])
  signal = tf.reshape(signal, [1, length, channels])
  return x + signal

def transformer_prepare_decoder(targets, hparams):
  """Prepare one shard of the model for the decoder.

  Args:
    targets: a Tensor.
    hparams: run hyperparameters

  Returns:
    decoder_input: a Tensor, bottom of decoder stack
    decoder_self_attention_bias: a Tensor, containing large negative values
    to implement masked attention and possibly baises for diagonal alignments
  """
  decoder_self_attention_bias = (
      common_attention.attention_bias_lower_triangle(tf.shape(targets)[1]))
  decoder_input = common_layers.shift_left_3d(targets)
  if hparams.pos == "timing":
    decoder_input = common_attention.add_timing_signal_1d(decoder_input)
  return (decoder_input, decoder_self_attention_bias)