Initialize last_epoch in _LRScheduler.__init__() (#20059)
Summary:
Class attributes preferably be explicitly initiated within
the __init__() call. Otherwise, overriding step() is
prone to bugs.
This patch partially reverts #7889
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20059
Differential Revision: D15195747
Pulled By: soumith
fbshipit-source-id: 3d1a51d8c725d6f14e3e91ee94c7bc7a7d6c1713
