Loads the data or reads it from cache.

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