unify linear benchmark (#28897)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28897
as title
Test Plan:
```
buck run mode/opt //caffe2/benchmarks/operator_benchmark/pt:linear_test
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short
# Benchmarking PyTorch: linear
# Mode: Eager
# Name: linear_N4_IN256_OUT128_cpu
# Input: N: 4, IN: 256, OUT: 128, device: cpu
Forward Execution Time (us) : 39.275
Reviewed By: hl475
Differential Revision: D18228070
fbshipit-source-id: 9c209eb74e574c6ef85ebcd78b824ef7d5e65dde
