def _copy_problem_hparams(p_hparams):
  """Use input modality, vocab, and space id for target."""
  p = p_hparams
  # Duplicate input modality.
  p.modality["targets"] = p.modality["inputs"]
  # Duplicate input vocab size.
  p.vocab_size["targets"] = p.vocab_size["inputs"]
  # Duplicate input vocabulary.
  p.vocabulary["targets"] = p.vocabulary["inputs"]
  # Duplicate input space ids.
  p.target_space_id = p.input_space_id
  # Mark that p was reversed.
  p.was_copy = True
Use input modality, vocab, and space id for target.
tensor2tensor/data_generators/problem.py