unify softmax benchmark (#28911)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28911
as title
Test Plan:
```
buck run mode/opt //caffe2/benchmarks/operator_benchmark/pt:softmax_test
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short
# Benchmarking PyTorch: Softmax
# Mode: Eager
# Name: Softmax_N4_C3_H256_W256_cpu
# Input: N: 4, C: 3, H: 256, W: 256, device: cpu
Forward Execution Time (us) : 17929.381
...
Reviewed By: hl475
Differential Revision: D18231517
fbshipit-source-id: 61f35849e1f4cf44cf09e60a7b618f8e9fc67b9c
