Base attention params.

def lstm_attention_base():
  """Base attention params."""
  hparams = lstm_seq2seq()
  hparams.add_hparam("attention_layer_size", hparams.hidden_size)
  hparams.add_hparam("output_attention", True)
  hparams.add_hparam("num_heads", 1)
  return hparams