Simplify adding static parameters and metric to MLFlow

[AHarouni]

Describe the solution you'd like
I would like to have an way to add static variable as well as running variables to the mlflow handle.
This was mainly for monai label training as in Project-MONAI/MONAILabel#1383 (comment)
the ask was to fix it in monai core since monai label simply uses the handle

[wyli]

I think you should first make it part of engine.state then fetch it here

MONAI/monai/handlers/mlflow_handler.py

Line 202 in e18097d

attrs = {attr: getattr(engine.state, attr, None) for attr in self.default_tracking_params}

@Nic-Ma @binliunls please help confirm the usage (converting this into discussion for now)