fix multiprocessing error

import time

import shutil

import multiprocessing

from multiprocessing.dummy import Pool

from ast import literal_eval as make_tuple

from typing import Any, Dict, Iterable, Iterator, List, Optional, Sequence, Tuple, Union

      # objects as an extra overhead. Also, as hideously as un-thread safe this looks,

      # we're actually protected by the GIL.

      # mp.dummy aliases ThreadPool to Pool

      pool = multiprocessing.dummy.Pool(1)

      pool = Pool(1)

      if batch_size is None:

        num_global_batches = num_shards