Add checkpoint artifact path prefix to MLflow logger

[benglewis]

What does this PR do?

Introduces a new checkpoint_path_prefix argument to MLFlowLogger which customizes the prefix for the model artifacts.

Fixes #20540

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Add a new checkpoint_path_prefix parameter to the MLflow logger.

• Modify src/lightning/pytorch/loggers/mlflow.py to include the new parameter in the MLFlowLogger class constructor and use it in the after_save_checkpoint method.
• Update the documentation in docs/source-pytorch/visualize/loggers.rst to include the new checkpoint_path_prefix parameter.
• Add a new test in tests/tests_pytorch/loggers/test_mlflow.py to verify the functionality of the checkpoint_path_prefix parameter and ensure it is used in the artifact path.

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📚 Documentation preview 📚: https://pytorch-lightning--20538.org.readthedocs.build/en/20538/

[lantiga]

Thanks a lot of the PR, I left a couple of comments but we're good to go.

Tests need fixing, can you take that @benglewis?

[lantiga]

thank you @benglewis , happy to merge once tests are fixed

[benglewis]

@lantiga I have commit a change that fixes the test when I ran it with pytest :)
This will be a great improvement for us in the next pytorch-lightning release

[benglewis]

@lantiga Do you need any additional changes here? 🙏

[lantiga]

Looks great, thanks @benglewis!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79%. Comparing base (1f5add3) to head (d892b4e).
Report is 2 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (1f5add3) and HEAD (d892b4e). Click for more details.

HEAD has 2689 uploads less than BASE

Flag	BASE (1f5add3)	HEAD (d892b4e)
gpu	2	0
pytest	314	0
lightning	468	18
cpu	623	24
python3.9	156	6
lightning_fabric	79	0
python3.10	78	3
python3.11	155	6
python3.12.7	234	9
pytorch2.1	117	9
pytest-full	311	24
pytorch_lightning	78	6
pytorch2.3	38	3
pytorch2.2.2	39	3
pytorch2.4.1	39	3
pytorch2.5.1	78	6

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20538     +/-   ##
=========================================
- Coverage      88%      79%     -9%     
=========================================
  Files         267      264      -3     
  Lines       23366    23312     -54     
=========================================
- Hits        20475    18365   -2110     
- Misses       2891     4947   +2056     

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[benglewis]

@lantiga I just updated the stale documentation as per @joncarter1 's comment. It should be ready for merge now 🙏

[benglewis]

@lantiga Do I need to do something to run the check/schema and required-jobs checks?

[benglewis]

@Borda @ethanwharris @tchaton @justusschock @lantiga Is there anything in particular holding this up?

[benglewis]

@Borda Is there anything that I need to change to get this PR merged?

[benglewis]

@Borda @ethanwharris @tchaton @justusschock @lantiga
I don't like "nagging" and such on GitHub PRs, and I understand and image that you are all extremely busy at PyTorch Lightning, I was just wondering what is preventing this PR from being merged :/