unify cat benchmark (#28893)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28893
as title
Test Plan:
```
buck run mode/opt //caffe2/benchmarks/operator_benchmark/pt:cat_test
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short
# Benchmarking PyTorch: cat
# Mode: Eager
# Name: cat_M256_N512_K1_dim0_cpu
# Input: M: 256, N: 512, K: 1, dim: 0, device: cpu
Forward Execution Time (us) : 78.607
Reviewed By: hl475
Differential Revision: D18227341
fbshipit-source-id: d383709a5aab600f99b37d07e4d4393645289101
