[Feature] AdaGC and SimplifiedAdEMAMix optimizers

[kozistr]

Problem (Why?)

Close #364

Solution (What/How?)

• implement AdaGC optimizer
• implement SimplifiedAdEMAMix optimizer

Other changes (bug fixes, small refactors)

N/A

Notes

N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (487200d) to head (771541a).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main      #366    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          112       113     +1     
  Lines         9118      9243   +125     
==========================================
+ Hits          9118      9243   +125     

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