mlflow is not required if not being used

[williamFalcon]

remove requirement to import mlflow

[williamFalcon]

@neggert users shouldn't have to install mlflow or comet if they're not using it.
No need to raise exception, it'll just crash and users will know

[neggert]

Yeah, was just about to send a PR when I saw this. It was originally this way, but got messed up in #395. I have a different fix that I like better because it still throws an error if you explicitly try to import e.g. CometLogger without Comet installed.

[Borda]

you do not need the e, PEP8 will complain that you define a variable which is not used

[Borda]

@williamFalcon @neggert I do not agree with ignoring the import error. when you try to import mlflow_logger.py directly and you do not have mlflow you should get an error message... The ignore is done in logging.__init__.py where you say the mlflow or any other is not needed.

[neggert]

@Borda I think #442 does what you're asking for.

[Borda]

@Borda I think #442 does what you're asking for.

That one is fine, but with this PR you change the meaning of ImportError completely...
It is like you go to sweet shop with optional desk with ice-screen, you want ice -cream, go to the desk and it is empty without any information... keeping the raise Exception gives you info that there is no ice-cream if you come to the deck but it does not border you if you come to sweet shop and you do not case about ice-cream...