Remove ST from torchsnapshot

[fduwjj]

As part of ShardedTensor deprecation, we start the cleanup for its use case in torch snapshot. This is the first PR for a series PR and want to get feedback from owner and the community to see if this is the right thing to do.

Summary:
As above.

Test plan:
Unit test.

[codecov]

Codecov Report

Merging #143 (496be7c) into main (087caae) will decrease coverage by 41.37%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main     #143       +/-   ##
===========================================
- Coverage   92.15%   50.78%   -41.37%     
===========================================
  Files          58       58               
  Lines        4408     4405        -3     
===========================================
- Hits         4062     2237     -1825     
- Misses        346     2168     +1822     

Impacted Files	Coverage Δ
torchsnapshot/io_preparer.py	67.21% <100.00%> (-29.62%)	⬇️

... and 48 files with indirect coverage changes

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[fduwjj]

Paused this for now and we need to verify with TorchRec on its usage.

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