ReduceLrOnPlateau: best=current -> best=copy(current) (#16364) (#16697)
Summary:
Fixes #16364
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16697
Differential Revision: D14680879
Pulled By: soumith
fbshipit-source-id: c50c22f3eacea4474fb3a04fe85fbf11d5a177c9
