Summary:
This adds a cosine schedule, based on:
 - "Bag of Freebies for Training Object Detection Neural Networks" https://arxiv.org/pdf/1902.04103.pdf
  - "SGDR: Stochastic Gradient Descent with Warm Restarts" https://arxiv.org/pdf/1608.03983.pdf

Reviewed By: ashwinb

Differential Revision: D14593944

fbshipit-source-id: f0f7eedee784fa4d0aebd092d718526bbf627d81