fix optimizer loop with frequencies

[awaelchli]

What does this PR do?

When extracting the optimizer loop #9191 , we broke the support for multiple optimizers that have different frequencies (deeply sorry for the inconvenience this has caused). This PR fixes that and adds a test.
No attached issue, found this while working on #9490 with @carmocca

Verified that the added test passes fine on 1.4.x branch.
Here is the failure we see on master:

        optimizer_sequence = [opt._optimizer.__class__.__name__ for opt in pl_optimizer_sequence]
>       assert optimizer_sequence == ["SGD", "SGD", "SGD", "Adam", "SGD", "SGD", "SGD", "Adam", "SGD", "SGD"]
E       AssertionError: assert ['SGD', 'SGD', 'SGD', 'SGD', 'SGD', 'SGD', 'SGD', 'SGD', 'SGD', 'SGD'] == ['SGD', 'SGD', 'SGD', 'Adam', 'SGD', 'SGD', 'SGD', 'Adam', 'SGD', 'SGD']

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[codecov]

Codecov Report

Merging #9507 (8367cc3) into master (1814334) will decrease coverage by 0%.
The diff coverage is 100%.

❗ Current head 8367cc3 differs from pull request most recent head d821ad3. Consider uploading reports for the commit d821ad3 to get more accurate results

@@          Coverage Diff           @@
##           master   #9507   +/-   ##
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- Coverage      93%     93%   -0%     
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  Files         180     180           
  Lines       15107   15096   -11     
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- Hits        14033   14011   -22     
- Misses       1074    1085   +11     

[carmocca]

Looks great other than my previous comment 👍

[tchaton]

LGMT !