Separate epoch validation from step validation (#5208)

* Seperate epoch validaton from step validation

* update system

* test

* baked logic in callbacks

* unbake logic in callbacks

* fix the call for scheduler

* use property

* pep

* correct rebase

* gitignore

* ref

* add tests

* fix

* add early stopping test

* trigger

* chlog

* rev

* 1.3

* log

* Apply suggestions from code review

Co-authored-by: Carlos Mocholí <[email protected]>

* Update pytorch_lightning/trainer/training_loop.py

* Update CHANGELOG.md

* Apply suggestions from code review

Co-authored-by: chaton <[email protected]>
Co-authored-by: Carlos Mocholí <[email protected]>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
Co-authored-by: Jirka Borovec <[email protected]>

(cherry picked from commit e429f97b670846bf0e0595074c4d3c4f152dcf27)