[NNC] Add Softplus operator (#64589)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64589
Adding softplus operator lowering for NNC. Enabling element wise fusion as well.
Test Plan: Added a test in test_jit_fuser.py
Reviewed By: bertmaher
Differential Revision: D30736449
fbshipit-source-id: 6c5fc3bceb5cef2322ecd4449f827e4af018ea93
