[DCP] Makes fsspec public (#121508)
Fixes #118033
Also removes `_checkpointer.py` class
original PR's:
- https://github.com/pytorch/pytorch/pull/121330
- https://github.com/pytorch/pytorch/pull/121329
We're also disabling `test_fsdp` since it is failing on random PR's
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121508
Approved by: https://github.com/fegin
