Fix self.optimizers() not returning a single LightningOptimizer

[carmocca]

What does this PR do?

When using deepspeed, the optimizers are wrapped and self.optimizers() returned a list. This means that changing the plugin needs a code change. This PR fixes it.

Found in #8071

Does your PR introduce any breaking changes? If yes, please list them.

• Users relying on the previous (bug) behaviour will get a TypeError.
• is_lightning_optimizer is removed. But seems like a silly function to have, isinstance is the pythonic check. Should be okay as the LightningOptimizer is internal and invisible to users.

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[codecov]

Codecov Report

Merging #8326 (434b6ad) into master (fc12fe7) will decrease coverage by 0%.
The diff coverage is 80%.

@@          Coverage Diff           @@
##           master   #8326   +/-   ##
======================================
- Coverage      92%     92%   -0%     
======================================
  Files         213     213           
  Lines       13808   13806    -2     
======================================
- Hits        12754   12739   -15     
- Misses       1054    1067   +13     

[tchaton]

LGTM ! Nice catch !

[kaushikb11]

LGTM!