How to use the tcav.cav.CAV.cav_key function in tcav

def load_cav_direction(self, c, r, bn, directory=None):
    """Loads an already computed cav.

    Args:
      c: concept name
      r: random concept name
      bn: bottleneck layer
      directory: where CAV is saved

    Returns:
      The cav instance
    """
    if directory is None:
      directory = self.cav_dir
    params = tf.contrib.training.HParams(model_type='linear', alpha=.01)
    cav_key = cav.CAV.cav_key([c, r], bn, params.model_type, params.alpha)
    cav_path = os.path.join(self.cav_dir, cav_key.replace('/', '.') + '.pkl')
    vector = cav.CAV.load_cav(cav_path).cavs[0]
    return np.expand_dims(vector, 0) / np.linalg.norm(vector, ord=2)