[Executorch] optimized sigmoid

Pull Request resolved: https://github.com/pytorch/executorch/pull/6522

basically use exp approximation using sleef instead of std::exp
ghstack-source-id: 254026289
@exported-using-ghexport

Differential Revision: [D64156864](https://our.internmc.facebook.com/intern/diff/D64156864/)

Co-authored-by: Kimish Patel <kimishpatel@fb.com>