remove beta defaulting in smooth_l1_loss_backward. added to the bc whitelist (#45588)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/45588
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D24024312
Pulled By: bdhirsh
fbshipit-source-id: 7246e5da741fbc5641deecaf057ae9a6e44e8c34
