[BE] Decorate unused functions with C10_UNUSED (#52378)
Summary:
This suppresses repeated warnings for every file that includes vec256 or
math.h:
```
../aten/src/ATen/native/Math.h:1095:15: warning: unused function 'calc_igamma' [-Wunused-function]
c10::BFloat16 calc_igamma<c10::BFloat16>(c10::BFloat16 a, c10::BFloat16 x) {
^
../aten/src/ATen/native/Math.h:1100:11: warning: unused function 'calc_igamma' [-Wunused-function]
c10::Half calc_igamma<c10::Half>(c10::Half a, c10::Half x) {
^
../aten/src/ATen/native/Math.h:1105:15: warning: unused function 'calc_igammac' [-Wunused-function]
c10::BFloat16 calc_igammac<c10::BFloat16>(c10::BFloat16 a, c10::BFloat16 x) {
^
../aten/src/ATen/native/Math.h:1110:11: warning: unused function 'calc_igammac' [-Wunused-function]
c10::Half calc_igammac<c10::Half>(c10::Half a, c10::Half x) {
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52378
Reviewed By: walterddr
Differential Revision: D26492412
Pulled By: malfet
fbshipit-source-id: c570c9beb9915c96fca297e0b88d0291937d3132
