Fix WandbLogger.watch() method annotation to accept None for the log parameter

[daturkel]

This is a comically small change that fixed an annoyance I was having. The wandb watch method has a log argument which can be a string ("gradients", "parameters", "all") OR None. You might provide None if you want to call watch with log_graph but don't want to log the gradients or parameters.

However, the WandBLogger watch method has a type annotation of str for the log parameter which is passed to the underlying wandb.watch log parameter. This PR simply changes that type annotation to Optional[str] so that None doesn't violate the type annotation.

The current WandBLogger docs are kind of more "tutorial style" rather than a full reference of all the methods on the class, so there isn't necessarily a change that needs to be made. However, I'm open to suggestions.

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📚 Documentation preview 📚: https://pytorch-lightning--19237.org.readthedocs.build/en/19237/

[codecov]

Codecov Report

Merging #19237 (53920c0) into master (ae1f78c) will decrease coverage by 34%.
The diff coverage is 100%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19237      +/-   ##
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- Coverage      83%      49%     -34%     
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  Files         445      437       -8     
  Lines       37301    37148     -153     
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- Hits        30990    18238   -12752     
- Misses       6311    18910   +12599     

[daturkel]

Thanks for the approvals!