Allow zero annealing epochs (#47579)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/47578.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47579
Reviewed By: H-Huang
Differential Revision: D25429403
Pulled By: vincentqb
fbshipit-source-id: c42fbcd71b46e07c672a1e9661468848ac16de38
