remove TrainerCallbackConfigMixin
🚀 Feature
Callbacks should be initialized and configured in their respective classes.
Motivation
The config mixin is opaque and reaches into callback objects which should be able to be self-contained.
Pitch
Move the configuration setup to the actual callback classes.
Alternatives
Keep the mixin to construct the objects and insert them into the callbacks list, but remove any of the other setup. The model checkpoint callback does a lot of configuration that should be happening within the class.