fix mypy typing errors in pytorch_lightning/strategies/strategy.py

[CyprienRicque]

What does this PR do?

Fixes mypy typing errors in pytorch_lightning/strategies/strategy.py in #13445.

• model becomes explicitly Optional[] in plugins functions such as optimizer_step and _after_closure
• self.accelerator becomes explicitly Optional[] in strategy classes
• The existence of self.accelerator, self.model and self.lightning_module is checked in strategy classes using assert when needed
• Variable output in training_step_end, validation_step_end and test_step_end has been renamed to step_output to stay consistent with other functions
• self.model has to be of type pl.LightningModule or _LightningPrecisionModuleWrapperBase to use training_step, validation_step, test_step and predict_step

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

None

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[awaelchli]

Great work. Allowed myself to make minor cosmetic edits :)

[codecov]

Codecov Report

Merging #13519 (65eed42) into master (85b0356) will decrease coverage by 10%.
The diff coverage is 86%.

@@            Coverage Diff            @@
##           master   #13519     +/-   ##
=========================================
- Coverage      86%      76%    -10%     
=========================================
  Files         330      330             
  Lines       25941    25971     +30     
=========================================
- Hits        22241    19745   -2496     
- Misses       3700     6226   +2526