Builds encoder for the given class labels.

    Args:
      data_dir: data directory

    Returns:
      An encoder for class labels.

def get_labels_encoder(self, data_dir):
    """Builds encoder for the given class labels.

    Args:
      data_dir: data directory

    Returns:
      An encoder for class labels.
    """
    label_filepath = os.path.join(data_dir, self.vocab_filename)
    return text_encoder.TokenTextEncoder(label_filepath)