Reinstate checkpoint_path_prefix in MLFlowLogger

[joncarter1]

What does this PR do?

Reinstates the optional checkpoint_path_prefix argument for the MLFlowLogger.

This was added in lightning==2.5.1 but removed in lightning==2.6.0.

Why?

lightning<2.5 logged model checkpoints in MLFlow under a hard-coded subfolder: model/checkpoints
lightning==2.5.0 changed this to log checkpoints under the root folder within a run.
This location change broke the MLOps tooling of myself and others who programatically retrieved these checkpoints.

The checkpoint_path_prefix argument added by @benglewis in #20538 meant that the previous behaviour could be restored. It was susceptible to a bug in MLFlow, so a patch in Lightning was introduced in #20669

However, both changes were yanked from the master branch:

• f6ef409
• 73b785a

The root issue was later fixed in MLFlow in mlflow/mlflow#16660 (MLFlow 3.6)

Closes #20932

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📚 Documentation preview 📚: https://pytorch-lightning--21432.org.readthedocs.build/en/21432/