replace add benchmark with add_ (#29050)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29050
as title
Test Plan:
```
buck run //caffe2/benchmarks/operator_benchmark/pt:add_test
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short
# Benchmarking PyTorch: add_
# Mode: Eager
# Name: add__M64_N64_K64_cpu
# Input: M: 64, N: 64, K: 64, device: cpu
Forward Execution Time (us) : 31475.766
Reviewed By: hl475
Differential Revision: D18265767
fbshipit-source-id: 7aaa04f5fa5b2dd58bbc1aa045693314032e0ff0
