consolidate callbacks and hooks (#950)

* 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

docs/source/callbacks.rst

@@ -44,8 +44,4 @@ Callback Class
         _del_model,
         _save_model,
         _abc_impl,
-        on_epoch_end,
-        on_train_end,
-        on_epoch_start,
         check_monitor_top_k,
-        on_train_start,