Args:
            df (DataFlow): the DataFlow to serialize.
            path (str): output tfrecord file.

def save(df, path):
        """
        Args:
            df (DataFlow): the DataFlow to serialize.
            path (str): output tfrecord file.
        """
        if os.environ.get('TENSORPACK_COMPATIBLE_SERIALIZE', 'msgpack') == 'msgpack':
            def _dumps(dp):
                return dumps(dp)
        else:
            def _dumps(dp):
                return dumps(dp).to_pybytes()

        size = _reset_df_and_get_size(df)
        with tf.python_io.TFRecordWriter(path) as writer, get_tqdm(total=size) as pbar:
            for dp in df:
                writer.write(_dumps(dp))
                pbar.update()