Fixed PreTrainedModel.from_pretrained(...) not passing cache_dir to PretrainedConfig.from_pretrained(...)
See #863 

It's not a beautiful solution, but neither is the practice of modifying incoming parameters via pop. 🤷‍♂ 
Indeed thanks. We'll subsume this PR with #866 which add a few other stuff.

I agree with you on the `pop` pattern. We'll move away from this when the first one of these two events happens: (i) google stop open-sourcing interesting new models or (ii) google stop using python 2 internally ;)<|||||>Okay! :+1: 