[nnc] Expose fast tanh/sigmoid (#50736)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50736
Exposes tanh and sigmoid to other backends
Test Plan: buck test caffe2/test/cpp/tensorexpr:tensorexpr -- "ATen.fast"
Reviewed By: bertmaher
Differential Revision: D25884911
fbshipit-source-id: f9a5286450331f60935cfd40bb23f4a4f4c1d087
