Snapshot selected globals and restore them in spawned process

[awaelchli]

What does this PR do?

Fixes #12685

The problem: Globals like torch deterministic flags or the seed get lost when spawning processes. This is caused because essentially the torch module gets re-imported in the child process, leading to globals being re-initialized. This is simply the nature of how mp.spawn works. This means it only affects ddp spawn (multiprocessing start method "spawn").

• Subprocess launch is not affected because all processes start from an identical state by design
• Fork is not affected because all children inherit the exact memory of the parent process

We can solve this by picking the globals we care about, save them, send them to the child process and restore them there before we launch the trainer run methods. Hence, this PR implements this directly in the launcher.

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @justusschock @kaushikb11 @awaelchli @akihironitta @rohitgr7

[akihironitta]

LGTM!

[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #13921 (eab9c16) into master (98f7326) will increase coverage by 15%.
The diff coverage is 93%.

@@            Coverage Diff            @@
##           master   #13921     +/-   ##
=========================================
+ Coverage      61%      76%    +15%     
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  Files         335      335             
  Lines       26305    26368     +63     
=========================================
+ Hits        16048    20137   +4089     
+ Misses      10257     6231   -4026