Add benchmark for add op. (#40059)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40059
This benchmark is added specifically for mobile to see if compiler is
autovectorizing and thus we have no advantage of neon backend for vec256
for add op.
Test Plan:
CI
Imported from OSS
Differential Revision: D22055146
fbshipit-source-id: 43ba6c4ae57c6f05d84887c2750ce21ae1b0f0b5
