[bug-fix] DDP and automatic_optimization=False (#4485)

* resolve bug

* add self._running_manual_optim

* update

* update tests

* update lightning module

* resolve bug

* update tests

* update

* resolve pep8

* update

* replace by `ddp_spawn`

* temporary fix

* update

* update

* move update to training_loop

* make both ddp_spawn

* introduce `manual_optimizer_step`

* update changelog

* added changelog wrong place

* add force_optimizer_step

* update docstring for tests

* update optimizer_step

* update zero_grad

* resolve flake8

* move update into manual_optimizer_step

* add zero_grad

* remove zero_grad tests

* remove manual_backward in AMP, it doesn't help

* update

* loosen tests

* update

* update doc

* add TODO

* Removed unnecessary get model from native amp

* Remove try except with pytest raise

* Add seed, clean up imports, remove try catch to reproduce error

* update code

* update test

* revert back

* formatting

* Update pytorch_lightning/core/lightning.py

Co-authored-by: Jirka Borovec <[email protected]>

Co-authored-by: SeanNaren <[email protected]>
Co-authored-by: Sean Naren <[email protected]>
Co-authored-by: Jirka Borovec <[email protected]>