unify batchnorm benchmark (#28889)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28889
as title
Test Plan:
```
buck run mode/opt //caffe2/benchmarks/operator_benchmark/pt:batchnorm_test
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short
# Benchmarking PyTorch: batchnorm
# Mode: Eager
# Name: batchnorm_M1_N256_K3136_cpu
# Input: M: 1, N: 256, K: 3136, device: cpu
Forward Execution Time (us) : 276.192
Reviewed By: hl475
Differential Revision: D18227180
fbshipit-source-id: d8abe56237bb84903315332a5ecdaa1dff613110
