Get hyper-parameters file path.

def _get_hparams_path():
  """Get hyper-parameters file path."""
  hparams_path = None
  if FLAGS.output_dir:
    hparams_path = os.path.join(FLAGS.output_dir, "hparams.json")
  else:
    tf.logging.warning(
        "--output_dir not specified. Hyper-parameters will be infered from"
        "--hparams_set and --hparams only. These may not match training time"
        "hyper-parameters.")
  return hparams_path