Adds the hparams used by get_standardized_layers.

def add_standard_attention_hparams(hparams):
  """Adds the hparams used by get_standardized_layers."""
  # All hyperparameters ending in "dropout" are automatically set to 0.0
  # when not in training mode.

  # hparams used and which should have been defined outside (in
  # common_hparams):
  # Global flags
  # hparams.mode
  # hparams.hidden_size
  # Pre-post processing flags
  # hparams.layer_preprocess_sequence
  # hparams.layer_postprocess_sequence
  # hparams.layer_prepostprocess_dropout
  # hparams.norm_type
  # hparams.norm_epsilon
  # Mixture-of-Expert flags
  # hparams.moe_hidden_sizes
  # hparams.moe_num_experts
  # hparams.moe_k
  # hparams.moe_loss_coef

  # Attention layers flags
  hparams.add_hparam("num_heads", 8)
  hparams.add_hparam("attention_key_channels", 0)
  hparams.add_hparam("attention_value_channels", 0)
  hparams.add_hparam("attention_dropout", 0.0)
  # Attention: Local
  hparams.add_hparam("attention_loc_block_length", 256)
  # Attention: Local (unmasked only): How much to look left.
  hparams.add_hparam("attention_loc_block_width", 128)
  # Attention: Memory-compressed
  hparams.add_hparam("attention_red_factor", 3)
  hparams.add_hparam("attention_red_type", "conv")
  hparams.add_hparam("attention_red_nonlinearity", "none")

  # Fully connected layers flags
  # To be more consistent, should use filter_size to also control the MOE
  # size if moe_hidden_sizes not set.
  hparams.add_hparam("filter_size", 2048)
  hparams.add_hparam("relu_dropout", 0.0)

  return hparams