feature: add sub_dir to CSV logger constructor

[Shoumik-Gandre]

What does this PR do?

Fixes #21035

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---

📚 Documentation preview 📚: https://pytorch-lightning--21045.org.readthedocs.build/en/21045/

[deependujha]

Nice work @Shoumik-Gandre.

Could you also update lightning/pytorch/loggers/csv_logs so this behavior is consistent across both Fabric and the Trainer?

didn't wanted to approve for now, and wait for test results

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87%. Comparing base (9ca360b) to head (d494e3b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21045   +/-   ##
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  Coverage      87%      87%           
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  Files         269      269           
  Lines       23501    23515   +14     
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+ Hits        20487    20505   +18     
+ Misses       3014     3010    -4     

[Shoumik-Gandre]

Thanks for the review, @deependujha - I've implemented the requested changes.

What changed

• Add sub_dir to pytorch CSVLogger to match fabric behavior.
• Ensure log_dir expands ~ and env vars while root_dir preserves the literal input (fixes test_csv_expand_home and env-var cases).
• Tests updated/added for home and env expansion, and sub_dir.

Why it helps

• Aligns path semantics across Fabric/PyTorch CSV loggers.
• Enables cleaner directory layout via sub_dir without breaking existing users.

Safety

• Backwards compatible (defaults unchanged).
• All tests pass locally for the touched modules.

@deependujha could you take another look and approve if this matches your expectations?
happy to loop in additional reviewers if needed

Thanks. looking forward to continued contributions!

✅All CI checks are green

[Shoumik-Gandre]

Hey @williamFalcon hope you’re doing well!
I wanted to check in on this PR. It already has the required number of approvals, but since you're listed as a code owner, I wanted to confirm if you could take a quick look and approve when you get a chance. Your review would help us get it merged smoothly. Thanks a lot! 🙏

[bhimrazy]

just missing the docstring here above.

        sub_dir: Sub-directory to group CSV logs. If a ``sub_dir`` argument is passed
            then logs are saved in ``/root_dir/name/version/sub_dir/``. Defaults to ``None`` in which case
            logs are saved in ``/root_dir/name/version/``.

[Borda]

cc: @Shoumik-Gandre

[Shoumik-Gandre]

Hey team. I was wondering if there's anything more I needed to do to get this PR merged?

[Borda]

Hey team. I was wondering if there's anything more I needed to do to get this PR merged?

you are fine, thank you for your patience, we are waiting Will's approval as he is solo owner of loggers