Applies multihead attention function which is parametrised for encoding.

  Args:
    x: input
    hparams: model hyper-parameters
    encoder_self_attention_bias: a bias tensor for use in encoder self-attention
    attention_dropout_broadcast_dims: Fpr noise broadcasting in the dropout
      layers to save memory during training
    save_weights_to: an optional dictionary to capture attention weights for
      visualization; the weights tensor will be appended there under a string
      key created from the variable scope (including name).
    make_image_summary: Whether to make an attention image summary.

  Returns:
    the output tensor

def transformer_encoder_attention_unit(x,
                                       hparams,
                                       encoder_self_attention_bias,
                                       attention_dropout_broadcast_dims,
                                       save_weights_to=None,
                                       make_image_summary=True):
  """Applies multihead attention function which is parametrised for encoding.

  Args:
    x: input
    hparams: model hyper-parameters
    encoder_self_attention_bias: a bias tensor for use in encoder self-attention
    attention_dropout_broadcast_dims: Fpr noise broadcasting in the dropout
      layers to save memory during training
    save_weights_to: an optional dictionary to capture attention weights for
      visualization; the weights tensor will be appended there under a string
      key created from the variable scope (including name).
    make_image_summary: Whether to make an attention image summary.

  Returns:
    the output tensor

  """

  with tf.variable_scope("self_attention"):
    y = common_attention.multihead_attention(
        common_layers.layer_preprocess(x, hparams),
        None,
        encoder_self_attention_bias,
        hparams.attention_key_channels or hparams.hidden_size,
        hparams.attention_value_channels or hparams.hidden_size,
        hparams.hidden_size,
        hparams.num_heads,
        hparams.attention_dropout,
        attention_type=hparams.self_attention_type,
        save_weights_to=save_weights_to,
        max_relative_position=hparams.max_relative_position,
        make_image_summary=make_image_summary,
        dropout_broadcast_dims=attention_dropout_broadcast_dims,
        hard_attention_k=hparams.hard_attention_k)
    x = common_layers.layer_postprocess(x, y, hparams)
  return x