Summary:
This diff does the following:
1. re-enable single_file_per_rank for FsspecWriter, as the issue of file slicing error is resolved because of [https://github.com/pytorch/pytorch/pull/99167]
2. remove sync_files from FsspecWriter as there is no fsspec equivalence.
3. remove the internal implementation of FsspecWriter/Reader, as it has been upstreamed to PyTorch OSS
4. keep the internal test for manifold inside internal as we can only test it in fb environment
5. consolidate test to remove duplicates
6. remove unnecessary TARGETS
Test Plan:
```
buck test @//mode/dev-nosan //caffe2/test/distributed/checkpoint/fb:test_fsspec_filesystem -- --print-passing-details
----------------------------------------------------------------------
Ran 1 test in 54.894s
OK
/usr/local/fbcode/platform010/lib/python3.8/tempfile.py:818: ResourceWarning: Implicitly cleaning up <TemporaryDirectory '/tmp/tmpzomokvh6'>
_warnings.warn(warn_message, ResourceWarning)
Buck UI: https://www.internalfb.com/buck2/4cb722a2-3ee7-48f2-a9ef-55ee6fb1a498
Test UI: https://www.internalfb.com/intern/testinfra/testrun/8725724447995201
Network: Up: 8.8 MiB Down: 1.5 GiB (reSessionID-04c29f56-ae94-4187-8a1a-c812f432674d)
Jobs completed: 209847. Time elapsed: 1:56.5s.
Cache hits: 100%. Commands: 85687 (cached: 85687, remote: 0, local: 0)
Tests finished: Pass 3. Fail 0. Fatal 0. Skip 0. Build failure 0
```
Differential Revision: D47266068
Pull Request resolved: https://github.com/pytorch/pytorch/pull/104724
Approved by: https://github.com/fegin, https://github.com/fduwjj
