Port elu_backward to structured (#58660)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/58660
Test Plan: Imported from OSS
Reviewed By: soulitzer
Differential Revision: D28572528
Pulled By: ezyang
fbshipit-source-id: 12265c287f178f9435d5d96f3bba49082d9e7f2c
