Fix passed metadata to mlflow logging

[wenfeiy-db]

Two fixes:

1. The passed mlflow_logging_config is an omegaconfig DictConfig, which is not serializable by yaml and will cause mlflow logging to fail if metadata is passed there. This PR convert it to python native dict in build_callbacks.
2. The current implementation doesn't set task key in metadata as long as metatdata is passed in mlflow_logging_config. However, it's possible for the client to pass a metatdata in mlflow_logging_config that doesn't include task key. In this case, we should keep other metadata and set the task.

[dakinggg]

Thanks! Could you take this opportunity to improve our test coverage too? :D I'm thinking just checking that things are called with the correct args here instead of just testing that the function is called

[wenfeiy-db]

@dakinggg Could you take another look? Tests added and CI is good!