Fix Trainer's log_dir method for CSVLogger

[ioangatop]

What does this PR do?

Fixes #18547

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📚 Documentation preview 📚: https://pytorch-lightning--18548.org.readthedocs.build/en/18548/

[codecov]

Codecov Report

Merging #18548 (2b84b41) into master (3a59462) will decrease coverage by 35%.
The diff coverage is 100%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #18548      +/-   ##
==========================================
- Coverage      85%      50%     -35%     
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  Files         427      419       -8     
  Lines       33396    33251     -145     
==========================================
- Hits        28437    16774   -11663     
- Misses       4959    16477   +11518     

[awaelchli]

Thanks for the fix. I think this is reasonable since CSVLogger follows the same design as TensorBoardLogger, so this LGTM. I also added a test for it.