unify chunk benchmark (#28892)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28892
as title
Test Plan:
```
buck run mode/opt //caffe2/benchmarks/operator_benchmark/pt:chunk_test
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short
# Benchmarking PyTorch: chunks
# Mode: Eager
# Name: chunks_M256_N512_chunks2_cpu
# Input: M: 256, N: 512, chunks: 2, device: cpu
Forward Execution Time (us) : 4.098
Reviewed By: hl475
Differential Revision: D18227499
fbshipit-source-id: 72268b7fe94a7d92d6e47f58f33902a33367c68b
