Make CSVLogger(name: ...) optional str

[mwip]

This patch adds Optional typing to the name parameter in CSVLogger reflecting the actual behavior. This increases coherence with other loggers and helps with type checking. Additionally documentation is improved.

This patch does NOT change the underlying behavior. Hence no additional tests were added. Tests already can be found in
tests/tests_pytorch/loggers/test_csv.py#L85-L90

Closes #19433

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📚 Documentation preview 📚: https://pytorch-lightning--19518.org.readthedocs.build/en/19518/

[awaelchli]

Thanks!

[codecov]

Codecov Report

Merging #19518 (6c9ab82) into master (a41528c) will decrease coverage by 29%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19518      +/-   ##
==========================================
- Coverage      82%      53%     -29%     
==========================================
  Files         450      445       -5     
  Lines       38253    38155      -98     
==========================================
- Hits        31454    20350   -11104     
- Misses       6799    17805   +11006