Simplify function pointers for apply_ternary_elementwise_fn (#5833)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5833

Cleaning up some of the required boilerplate. I updated op_clamp and op_where, but continued to not optimize op_where for size/build time. Ideal usage optimizing for size/build time would look like op_clamp.
ghstack-source-id: 246132259
exported-using-ghexport

Reviewed By: manuelcandales

Differential Revision: D63790004

fbshipit-source-id: e3e3f29587b9761ff042a5866111b1dc7e864f11