Return bool from snapshot restore_from_latest

[ananthsub]

Summary:
As title, this indicates to callers if the states were restored or not. This allows callers to do logic like:

restored = TorchSnapshotSaver.restore_from_latest(...)

# no prior checkpoints, so initialize weights for the first attempt
if not restored:
    <initialization logic>

Reviewed By: daniellepintz

Differential Revision: D48207346

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D48207346

[codecov]

Codecov Report

Merging #498 (732fbd6) into master (a9ad674) will increase coverage by 0.00%.
The diff coverage is 80.76%.

@@           Coverage Diff           @@
##           master     #498   +/-   ##
=======================================
  Coverage   86.91%   86.91%           
=======================================
  Files         102      102           
  Lines        8337     8340    +3     
=======================================
+ Hits         7246     7249    +3     
  Misses       1091     1091           

Files Changed	Coverage Δ
...orchtnt/framework/callbacks/torchsnapshot_saver.py	89.26% <66.66%> (ø)
...ts/framework/callbacks/test_torchsnapshot_saver.py	89.26% <100.00%> (+0.10%)	⬆️

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