softplus_backward: remove output arg (#70296)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/69042
Tested with OpInfo
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70296
Reviewed By: mikaylagawarecki
Differential Revision: D33349227
Pulled By: albanD
fbshipit-source-id: edeb35cb19ab4434d39df93d4536cb07679218b5
