move get_active_optimizers to utilities

[awaelchli]

What does this PR do?

Part of #7938

Moves the get_active_optimizers() method to the utilities. It is currently used by multiple loops and after #9490 only in one loop, but there is no reason why it should be a method on any of the loops as loops don't own optimizers, nor should loop customization make any assumptions which loop has this method.

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Did you have fun?

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[codecov]

Codecov Report

Merging #9581 (2f8d4b9) into master (2b2537d) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #9581   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         179     179           
  Lines       15329   15331    +2     
======================================
+ Hits        14222   14224    +2     
  Misses       1107    1107           

[tchaton]

LGTM !