Bring back math_silu_backward which works for all backends. (#49439)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/49439
Test Plan: Imported from OSS
Reviewed By: nikithamalgifb, ngimel
Differential Revision: D25594129
Pulled By: ailzhang
fbshipit-source-id: 627bbea9ba478ee3a8edcc6695abab6431900192
