Add `AdamW` to C++ frontend (#40009)
Summary:
Slightly modified Adam, following the python implementation, and the `ProducesPyTorchValues` tests pass. I had a problem with another test though (see commit c1a6241676ab84fc531c1c3a10f964aa5704092e), it seems that optimizing for two steps with the same optimizer vs optimizing for two steps using freshly initialized objects will produce the same output.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40009
Differential Revision: D22096053
Pulled By: glaringlee
fbshipit-source-id: a31a8f5488cb37c53752ddf15436efabdba67dc4
