Cheap model.

  on lm1b_32k:
     45M params
     2 steps/sec on  [GeForce GTX TITAN X]

  Returns:
    an hparams object.

def attention_lm_small():
  """Cheap model.

  on lm1b_32k:
     45M params
     2 steps/sec on  [GeForce GTX TITAN X]

  Returns:
    an hparams object.
  """
  hparams = attention_lm_base()
  hparams.num_hidden_layers = 4
  hparams.hidden_size = 512
  hparams.filter_size = 2048
  hparams.layer_prepostprocess_dropout = 0.5
  return hparams