What does this PR do?

• Allows to reuse any existing mlflow run that may be active before Trainer is initialized. Contrary to the discussion 🤗 Transformers **Trainer** API raises exception on train if triggered from an already started ML Flow run. #15663 the choice of creating a nested run is left to user since user now has the ability to explicitly start a nested run if they require it before calling the Trainer and reuse it for logging.
• Additionally, option to disable MLFLow Callback through environment variable. Useful when user wishes to purely control the flow outside transformers library context

Fixes #15663 #11115 (issue)

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@noise-field @sgugger
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