Release 2.3.0 broke custom iterable datasets
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Apparently, `fsspec` does not allow access to attribute-based modules anymore, such as `fsspec.async`.

However, this is a fairly simple fix:
- Change the import to: `from fsspec import asyn`;
- Change line 18 to: `asyn.iothread[0] = None`;
- Change line 19 to `asyn.loop[0] = None`. Hi! I think it's easier to replace `import fsspec` with `import fsspec.asyn` and leave the rest unchanged. @gugarosa Are you interested in submitting a PR? Perfect, it is even better!

Just submitted the PR: #4630.

Thank you!
## Describe the bug
Trying to iterate examples from custom iterable dataset fails to bug introduced in `torch_iterable_dataset.py` since the release of 2.3.0. 

## Steps to reproduce the bug
```python
next(iter(custom_iterable_dataset))
```

## Expected results
`next(iter(custom_iterable_dataset))` should return examples from the dataset

## Actual results
```
/usr/local/lib/python3.7/dist-packages/datasets/formatting/dataset_wrappers/torch_iterable_dataset.py in _set_fsspec_for_multiprocess()
     16     See https://github.com/fsspec/gcsfs/issues/379
     17     """
---> 18     fsspec.asyn.iothread[0] = None
     19     fsspec.asyn.loop[0] = None
     20 

AttributeError: module 'fsspec' has no attribute 'asyn'
```

## Environment info
- `datasets` version: 2.3.0
- Platform: Linux-5.4.188+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.7.13
- PyArrow version: 8.0.0
- Pandas version: 1.3.5

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