[Feat] Add auto_restart for fault tolerant training

[tchaton]

What does this PR do?

Once a signal.SIGUSR1 is being sent to Lightning, the Trainer should exit to the next most reproducible place. The Trainer would be aware of this as _terminate_gracefully would be equal to True.

This PR adds the following:

• Logic to the exit the loops using an ExitGracefullyException exception once safe places are reached
• Save a fault tolerant checkpoint
• Automatically reload the fault tolerant checkpoint if .pl_auto_save.ckpt files exists.

Part of #9130

Does your PR introduce any breaking changes? If yes, please list them.

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[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[codecov]

Codecov Report

Merging #9722 (5c9d28d) into master (60d4cc4) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9722    +/-   ##
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- Coverage      93%     89%    -4%     
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  Files         177     177            
  Lines       15390   15514   +124     
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- Hits        14264   13751   -513     
- Misses       1126    1763   +637