Base hparams for TransformerRevnet.

def transformer_revnet_big():
  """Base hparams for TransformerRevnet."""
  hparams = transformer_revnet_base()

  # The TransformerRevnet uses significantly less memory than the Transformer.
  # Increase batch size and model size.
  hparams.batch_size *= 2
  hparams.hidden_size *= 2
  hparams.num_heads *= 2
  hparams.num_hidden_layers += 1
  return hparams