Add fsspec to requirements.txt (#96532)
Need this package to support enable distributed checkpoint for different backends.
Fsspec package size:
```
du -h /fsx/users/irisz/conda/envs/pytorch/lib/python3.9/site-packages/fsspec-2023.3.0-py3.9.egg
264K /fsx/users/irisz/conda/envs/pytorch/lib/python3.9/site-packages/fsspec-2023.3.0-py3.9.egg/fsspec/__pycache__
58K /fsx/users/irisz/conda/envs/pytorch/lib/python3.9/site-packages/fsspec-2023.3.0-py3.9.egg/fsspec/implementations/__pycache__
377K /fsx/users/irisz/conda/envs/pytorch/lib/python3.9/site-packages/fsspec-2023.3.0-py3.9.egg/fsspec/implementations
1017K /fsx/users/irisz/conda/envs/pytorch/lib/python3.9/site-packages/fsspec-2023.3.0-py3.9.egg/fsspec
96K /fsx/users/irisz/conda/envs/pytorch/lib/python3.9/site-packages/fsspec-2023.3.0-py3.9.egg/EGG-INFO
1.2M /fsx/users/irisz/conda/envs/pytorch/lib/python3.9/site-packages/fsspec-2023.3.0-py3.9.egg
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96532
Approved by: https://github.com/osalpekar
