Add softplus support to autodiff (#63942)
Summary:
Add softplus definition to autodiff.
cc gmagogsfm
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63942
Reviewed By: ngimel
Differential Revision: D31397158
Pulled By: eellison
fbshipit-source-id: f7db547370f82e5e282505c3c8415fb4fbd86d54
