Add base for sgd optimizer (#3496)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3496

This adds the sgd_optimizer header to executorch. would appreciate some thoughts on where to place this file.

Reviewed By: JacobSzwejbka

Differential Revision: D56888378

fbshipit-source-id: 17d6bb3975ae2d58aee911ee91a3ff07acbc6850