C++ API parity: SELU (#27434)
Summary:
Adds `SELU` functional and module support for the C++ API.
Issue: https://github.com/pytorch/pytorch/issues/25883
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27434
Differential Revision: D17782762
Pulled By: yf225
fbshipit-source-id: 96c7ce84b9baf9e219a63e631929b8997ba6f3f0
