Add typing for LightningOptimizer

[carmocca]

What does this PR do?

Part of #7037

Also some docs changes

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

None

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[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[codecov]

Codecov Report

Merging #9990 (10f9f26) into master (3f355d0) will decrease coverage by 4%.
The diff coverage is 85%.

@@           Coverage Diff           @@
##           master   #9990    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         179     179            
  Lines       15822   15815     -7     
=======================================
- Hits        14660   14031   -629     
- Misses       1162    1784   +622     

[kandluis]

I believe we need to add a third overload here for this to work as expected.

Eg.

@overload
def optimizer(self, use_pl_optimizer: bool) -> Union[...]

See the error that is raised as currently written here: https://mypy-play.net/?mypy=latest&python=3.8&gist=183e84ea2331260b77cf58c0bfbf1151

[carmocca]

You are correct, this only works if True or False are explicitly passed.

I'll open a PR with the fix.