Issue: Multiprocessing windows error

Comment:





I was able to copy some of the shell 
This is repeating every half second
Win 10, Anaconda with python 3.8, datasets installed from main branche
```

  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 287, in _fixup_main_from_path
    _check_not_importing_main()
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 116, in spawn_main
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 134, in _check_not_importing_main
    main_content = runpy.run_path(main_path,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 265, in run_path
    exitcode = _main(fd, parent_sentinel)
    raise RuntimeError('''
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 125, in _main
RuntimeError:
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.    return _run_module_code(code, init_globals, run_name,
    prepare(preparation_data)

  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 97, in _run_module_code
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 236, in prepare
    _run_code(code, mod_globals, init_globals,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 87, in _run_code
    _fixup_main_from_path(data['init_main_from_path'])
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 287, in _fixup_main_from_path
    exec(code, run_globals)
  File "F:\Codes\Python Apps\asr\test.py", line 6, in <module>
    updated_dataset = dataset.map(lambda example: {'sentence1': 'My sentence: ' + example['sentence1']}, num_proc=4)
    main_content = runpy.run_path(main_path,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\datasets\arrow_dataset.py", line 1370, in map
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 265, in run_path
    with Pool(num_proc, initargs=(RLock(),), initializer=tqdm.set_lock) as pool:
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\context.py", line 119, in Pool
    return _run_module_code(code, init_globals, run_name,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 97, in _run_module_code
    _run_code(code, mod_globals, init_globals,
    return Pool(processes, initializer, initargs, maxtasksperchild,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\runpy.py", line 87, in _run_code
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\pool.py", line 212, in __init__
    exec(code, run_globals)
  File "F:\Codes\Python Apps\asr\test.py", line 6, in <module>
    self._repopulate_pool()
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\pool.py", line 303, in _repopulate_pool
    updated_dataset = dataset.map(lambda example: {'sentence1': 'My sentence: ' + example['sentence1']}, num_proc=4)
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\datasets\arrow_dataset.py", line 1370, in map
    return self._repopulate_pool_static(self._ctx, self.Process,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\pool.py", line 326, in _repopulate_pool_static
    with Pool(num_proc, initargs=(RLock(),), initializer=tqdm.set_lock) as pool:
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\context.py", line 119, in Pool
    w.start()
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\process.py", line 121, in start
    return Pool(processes, initializer, initargs, maxtasksperchild,
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\pool.py", line 212, in __init__
    self._popen = self._Popen(self)
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\context.py", line 327, in _Popen
    self._repopulate_pool()
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\pool.py", line 303, in _repopulate_pool
    return Popen(process_obj)
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\popen_spawn_win32.py", line 45, in __init__
    return self._repopulate_pool_static(self._ctx, self.Process,
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\pool.py", line 326, in _repopulate_pool_static
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 154, in get_preparation_data
    _check_not_importing_main()
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\spawn.py", line 134, in _check_not_importing_main
    w.start()
  File "C:\Users\flozi\anaconda3\envs\wav2vec\lib\site-packages\multiprocess\process.py", line 121, in start
    raise RuntimeError('''
RuntimeError:
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...
```