unify split benchmark (#28912)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28912
as title
Test Plan:
```
buck run mode/opt //caffe2/benchmarks/operator_benchmark/pt:split_test
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short
# Benchmarking PyTorch: split
# Mode: Eager
# Name: split_M256_N512_parts2_cpu
# Input: M: 256, N: 512, parts: 2, device: cpu
Forward Execution Time (us) : 3.434
Reviewed By: hl475
Differential Revision: D18231542
fbshipit-source-id: 84898db55996aa3faf156d4fb14f32d6db780e7a
