Release 2.3.0 broke custom iterable datasets
## 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
