* consolidate callbacks and hooks
* ensure callbacks recieve proper arg types
* remove model from init callback events
* clean up early stopping event
* update changelog
* remove on_fit_start and on_fit_end
* fix args for on_init_start and on_init_end
* handle case where early stopping is not used
* show all callback methods
* wrap checkpoint callback logic into proper class
* fix check for main process in checkpoint callback
* move callbacks test to separate file
* refactor arg checks
* get model and call hook on same line
* define trainer_options dict in one call
* add more asserts to callback test
Jeremy Jordan
705e576417