Use parallel version of SGD optimizer
Use parallel version of SGD optimizer

Summary:
Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/4633

X-link: https://github.com/facebookresearch/d2go/pull/406

Profiling yielded the training could benefit from using the parallel version of the SGD optimizer.
Passing `foreach=True` to enable.

Reviewed By: tglik

Differential Revision: D40798214

fbshipit-source-id: aa098d1fbbece0862bc9343df761765b0c3b15da

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2