Do not compile FP16 arith internally (#121844)
Also, decorate unused args with `C10_UNUSED` to fix linter warnings
Test Plan: `buck2 build -c fbcode.arch=aarch64 //caffe2:ATen-cpu`
Differential Revision: D54870507
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121844
Approved by: https://github.com/osalpekar
