Add three-phase option to OneCycleLR (#42715)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/40362
The new `three_phase` option provides a way of constructing schedules according to the scheme recommended in [Super-Convergence: Very Fast Training of Neural Networks Using Large Learning Rates](https://arxiv.org/abs/1708.07120).
Note that this change maintains backwards compatibility, and as a result the default behaviour of OneCycleLR remains quite counter-intuitive.
vincentqb
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42715
Reviewed By: heitorschueroff
Differential Revision: D24289744
Pulled By: vincentqb
fbshipit-source-id: e4aad87880716bb14613c0aa8631e43b04a93e5c
