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

def save(df, path):
        """
        Args:
            df (DataFlow): the DataFlow to serialize.
            path (str): output npz file.
        """
        buffer = []
        size = _reset_df_and_get_size(df)
        with get_tqdm(total=size) as pbar:
            for dp in df:
                buffer.append(dp)
                pbar.update()
        np.savez_compressed(path, buffer=np.asarray(buffer, dtype=np.object))