fix: Incorrect URI Prefix Stripping in MLFlowLogger

[awindmann]

What does this PR do?

Addresses the issue described in issue 20279. Essentially, str.lstrip() is used incorrectly, causing the MLFlowLogger to log in the wrong location under certain conditions. This can be fixed by stripping the uri prefix with another function. As str.removeprefix() was only introduced with Python 3.9.0, this pull request uses a more general method to strip the string, which should be okay because the method is contained by the if statement.

Background

The save_dir property in the MLFlowLogger class currently uses str.lstrip() to remove the LOCAL_FILE_URI_PREFIX ("file:") from the tracking URI. This method does not strip the prefix, but all characters that match those in the prefix ("f", "i", "l", "e", ":"). Thus, setting save_dir="logs" would be incorrectly modified to "ogs", leading to the creation of an "ogs" folder where logging subsequently occurs.

Minimal Example

from pytorch_lightning.loggers import MLFlowLogger

logger = MLFlowLogger(
    save_dir='logs'
)
print(logger.save_dir)  # ogs

Fixes #20279

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📚 Documentation preview 📚: https://pytorch-lightning--20365.org.readthedocs.build/en/20365/