Fix the Division by Zero Bug of CosineAnnealingLR (#19180)
Summary:
Added the formula for the corner case. Updated unit tests.
Fixes #17913
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19180
Differential Revision: D14942023
Pulled By: ezyang
fbshipit-source-id: 167c109b97a7830d5b24541dc91e4788d531feec
