Feat: support reusable instance of ModelCheckpoint

[ScarWar]

What does this PR do?

Fixes cross-validation folds checkpoint saving behavior

I'm using the great library https://github.com/SkafteNicki/pl_crossvalidate to cross validate in my project. The library is overriding some of the internal behavior of the trainer and the logs directory.

Expected behavior

When new fold is created, the checkpoint path should be changed to the new directory

Current behavior

The checkpoint path is resolved once during the first fold then it is short circuited and therefore never resolving to the new fold directory.

Proposed fix

New variable to save whether the dirpath parameter was passed by the user or was resolved from logger

The following links the related issue to the PR (https://docs.github.com/en/free-pro-team@latest/github/managing-your-work-on-github/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword)
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Fixes #20092

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