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 biases for diagonal alignments

def attention_lm_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 biases for diagonal alignments
  """
  if hparams.prepend_mode == "prepend_inputs_full_attention":
    decoder_self_attention_bias = (
        common_attention.attention_bias_prepend_inputs_full_attention(
            common_attention.embedding_to_padding(targets)))
  else:
    decoder_self_attention_bias = (
        common_attention.attention_bias_lower_triangle(
            common_layers.shape_list(targets)[1]))
  decoder_input = common_layers.shift_right_3d(targets)
  if hparams.pos == "timing":
    decoder_input = common_attention.add_timing_signal_1d(decoder_input)
  return (decoder_input, decoder_self_attention_bias)