Add new `checkpoint_artifact_path_prefix` argument `MLFlowLogger` to customize artifact path for model checkpoints

[benglewis]

Description & Motivation

See #19394 (comment)
PyTorch Lightning's MLFlowLogger changed its behavior and there are things that are inconvenient about the current implementation.

Pitch

I have opened a PR to fix this as per above.

Alternatives

No response

Additional context

No response

cc @lantiga @Borda

[lantiga]

Thank you for raising the issue @benglewis, in retrospect it was a breaking change.

Do you think we should make the default behavior go back to what it was, with the new added flexibility?

[benglewis]

@lantiga Sure. Should I adjust my PR to enact that change?