Batch MLFlowLogger requests

[schmidt-jake]

What does this PR do?

Fixes #15900.

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None

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[schmidt-jake]

To avoid import errors, I hid some mlflow.entities imports inside the test bodies. If someone could shine some light on how to better structure (or mock) things, I will be happy to make changes!

EDIT: thanks @awaelchli!

[awaelchli]

To avoid import errors, I hid some mlflow.entities imports inside the test bodies. If someone could shine some light on how to better structure (or mock) things, I will be happy to make changes!

Your solution would work assuming the package mlflow is installed. However, the tests there should run with mocked mlflow objects without the package being installed. I pushed a commit to demonstrate how to do it.

[schmidt-jake]

@Borda @carmocca looks like tests are all passing now! Could you please review?