trainer.check

[gianscarpe]

Hi pytorch-lightning friends! :) I'd like to start a discussion about trainer.check api.
Basically, this API should check that all the user-define classes (models, data, callbacks, ...) are programmatically sound. I propose to use inspect to check for function correctness, here's my PR proposal at #3244

[carmocca]

Locking in favor of #6029