def g(x):
    """g(x) for reversible layer, feed-forward layer."""
    old_hid_size = hparams.hidden_size
    hparams.hidden_size = old_hid_size // 2
    with tf.variable_scope("ffn"):
      y = transformer.transformer_ffn_layer(
          common_layers.layer_preprocess(x, hparams), hparams)
      y = common_layers.layer_postprocess(x, y, hparams)
    hparams.hidden_size = old_hid_size
    return y
g(x) for reversible layer, feed-forward layer.
tensor2tensor/models/research/transformer_revnet.py