Applies a feed-forward function which is parametrised for encoding.

  Args:
    x: input
    hparams: model hyper-parameters
    nonpadding_mask: optional Tensor with shape [batch_size, encoder_length]
    indicating what positions are not padding.  This is used
    to mask out padding in convoltutional layers.  We generally only
    need this mask for "packed" datasets, because for ordinary datasets,
    no padding is ever followed by nonpadding.
    pad_remover: to mask out padding in convolutional layers (efficiency).

  Returns:
    the output tensor

def transformer_encoder_ffn_unit(x,
                                 hparams,
                                 nonpadding_mask=None,
                                 pad_remover=None):
  """Applies a feed-forward function which is parametrised for encoding.

  Args:
    x: input
    hparams: model hyper-parameters
    nonpadding_mask: optional Tensor with shape [batch_size, encoder_length]
    indicating what positions are not padding.  This is used
    to mask out padding in convoltutional layers.  We generally only
    need this mask for "packed" datasets, because for ordinary datasets,
    no padding is ever followed by nonpadding.
    pad_remover: to mask out padding in convolutional layers (efficiency).

  Returns:
    the output tensor
  """

  with tf.variable_scope("ffn"):
    if hparams.transformer_ffn_type == "fc":
      y = transformer.transformer_ffn_layer(
          common_layers.layer_preprocess(x, hparams),
          hparams,
          pad_remover,
          conv_padding="SAME",
          nonpadding_mask=nonpadding_mask)

    if hparams.transformer_ffn_type == "sepconv":
      assert nonpadding_mask is not None, (
          "The nonpadding_mask should be provided, otherwise the model uses "
          "the leaked padding information to estimate the length!")
      y = common_layers.sepconv_relu_sepconv(
          common_layers.layer_preprocess(x, hparams),
          filter_size=hparams.filter_size,
          output_size=hparams.hidden_size,
          first_kernel_size=(3, 1),
          second_kernel_size=(5, 1),
          padding="SAME",
          nonpadding_mask=nonpadding_mask,
          dropout=hparams.relu_dropout)

    x = common_layers.layer_postprocess(x, y, hparams)

  return x