Improve callback system
Following the new callback system here are a few things we might want to do proposed by people who participated in #889 and #896.


 Consolidate model hooks and callbacks system. Make the calls at the same moment: #889 (comment)


 Fix on_train_begin() being called in the early stopping callbacks: #889 (comment)


 Assert pl_module and trainer are excepted object instances in the callback system train tests: #889 (comment)


 Incorporate existing callbacks into the new callback system.


 Edit the changelog


ping @Borda @williamFalcon @jeremyjordan @awaelchli