[sgd] fp16 (apex) and scheduler support + move examples page (#7061)

* Init fp16

* fp16 and schedulers

* scheduler linking and fp16

* to fp16

* loss scaling and documentation

* more documentation

* add tests, refactor config

* moredocs

* more docs

* fix logo, add test mode, add fp16 flag

* fix tests

* fix scheduler

* fix apex

* improve safety

* fix tests

* fix tests

* remove pin memory default

* rm

* fix

* Update doc/examples/doc_code/raysgd_torch_signatures.py

* fix

* migrate changes from other PR

* ok thanks

* pass

* signatures

* lint'

* Update python/ray/experimental/sgd/pytorch/utils.py

* Apply suggestions from code review

Co-Authored-By: Edward Oakes <ed.nmi.oakes@gmail.com>

* should address most comments

* comments

* fix this ci

* fix tests'

* testmode

Co-authored-by: Edward Oakes <ed.nmi.oakes@gmail.com>

doc/source/index.rst

@@ -281,7 +281,6 @@ Getting Involved
 
    raysgd/raysgd.rst
    raysgd/raysgd_pytorch.rst
-   raysgd/raysgd_pytorch_examples.rst
    raysgd/raysgd_tensorflow.rst
    raysgd/raysgd_ref.rst