Add SGDR(Stochastic Gradient Descent with Warm Restarts) scheduler (#17226)
Summary:
Because of merge error with master in #15042, open a new PR for ezyang.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17226
Differential Revision: D14418145
Pulled By: mrshenli
fbshipit-source-id: 099ba225b28e6aba71760b81b2153ad1c40fbaae
