Add checkpoint artifact path prefix to MLflow logger

[benglewis]

What does this PR do?

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

Fixes #20540

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Add a new checkpoint_artifact_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_artifact_path_prefix parameter.
• Add a new test in tests/tests_pytorch/loggers/test_mlflow.py to verify the functionality of the checkpoint_artifact_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