Update old scheduler example usage (#31358)
Summary:
Update the old example usage in CosineAnnealingWarm, `scheduler.step()` should be called after `optimizer.step()`.
https://github.com/pytorch/pytorch/issues/20028#issuecomment-566061580
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31358
Differential Revision: D19199311
Pulled By: vincentqb
fbshipit-source-id: cb29b95f8277d2dfa75ec2a83c1af03a5c9c9a69
