multiple optimizer restart with fault-tolerant training

[awaelchli]

What does this PR do?

Adds necessary changes and tests for restarting a run that failed at a training_step when multiple optimizers are involved. Note: if the user has interactions between the different training_step calls during the loop over the optimizers, this will not work or silently lead to different results, as it is the case with all other places were fault-tolerant takes place.

Part of #9130

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Did you have fun?

I made sure I had fun coding 🙃

[carmocca]

🚀

[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #9537 (b37730b) into master (5a846d4) will decrease coverage by 0%.
The diff coverage is 67%.

@@          Coverage Diff           @@
##           master   #9537   +/-   ##
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- Coverage      93%     93%   -0%     
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  Files         180     180           
  Lines       15090   15092    +2     
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+ Hits        14019   14020    +1     
- Misses       1071    1072    +1