[air/mlflow] Flatten config and metrics before passing to mlflow

[krfricke]

Why are these changes needed?

Metrics and parameters are passed as-is to mlflow e.g. in the MlFlowCallback. However, mlflow can't deal with nested dicts.

Instead, we should flatten these dicts before passing them over. This leads to the desired result:

Related issue number

Closes #34857

Checks

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    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
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• Testing Strategy
  • Unit tests
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  • This PR is not tested :(

[GokuMohandas]

Tested and worked like a charm, thanks @krfricke!

[amogkam]

nice, thanks!