Use Wandb experiment's files directory as save directory

[fschlatt]

What does this PR do?

Fixes #17654

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---

📚 Documentation preview 📚: https://pytorch-lightning--21462.org.readthedocs.build/en/21462/

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79%. Comparing base (027455b) to head (2644173).
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (027455b) and HEAD (2644173). Click for more details.

HEAD has 2271 uploads less than BASE

Flag	BASE (027455b)	HEAD (2644173)
python3.10	54	3
cpu	537	29
lightning	270	15
pytest	269	0
lightning_fabric	134	0
python3.12	161	8
python3.12.7	160	9
python3.11	108	6
python	54	3
pytorch_lightning	133	14
pytorch2.6	26	3
pytest-full	268	29
pytorch2.1	54	6
pytorch2.9	26	2
pytorch2.3	27	3
pytorch2.8	27	3
pytorch2.2.2	27	3
pytorch2.5.1	27	3
pytorch2.4.1	27	3
pytorch2.7	27	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #21462     +/-   ##
=========================================
- Coverage      87%      79%     -8%     
=========================================
  Files         270      267      -3     
  Lines       24059    24009     -50     
=========================================
- Hits        20862    18957   -1905     
- Misses       3197     5052   +1855     

[fschlatt]

This PR is not ready to merge. I've reverted fixing the tests, because I misinterpreted what the save_dir argument did. Implementing proper tests is a bit more involved and is something I will try to solve soon.