docs: def _get_raw_data(subset):
    """Loads the data or reads it from cache."""
    raw_data = _LOADED.get(subset)
    if raw_data is not None:
      return raw_data, _LOADED["vocab"]
    else:
      train_data, valid_data, test_data, vocab = ptb_reader.ptb_raw_data(
          FLAGS.data_path)
      _LOADED.update({
          "train": np.array(train_data),
          "valid": np.array(valid_data),
          "test": np.array(test_data),
          "vocab": vocab
      })
      return _LOADED[subset], vocab