def _get_encoder_data_shapes(self, bucket_key: int, batch_size: int) -> List[mx.io.DataDesc]:
        """
        Returns data shapes of the encoder module.

        :param bucket_key: Maximum input length.
        :return: List of data descriptions.
        """
        return [mx.io.DataDesc(name=C.SOURCE_NAME,
                               shape=(batch_size, bucket_key, self.num_source_factors),
                               layout=C.BATCH_MAJOR)]