Advanced GPU Documentation (#7259)

* Added advanced gpu section

* Small changes

* Better documentation

* Address code review

* Add warning about using trainer.model, clean up some of the examples

* Add section for ddp, remove references and old sequential documentation

* Remove Fully Sharded documentation for now

* Apply suggestions from code review

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

* Address code review

* Address code review

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

Author: 3 people