report p50 time instead of avg (#28722)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28722
as title
Test Plan:
```buck run mode/opt caffe2/benchmarks/operator_benchmark:benchmark_all_test -- --operator sigmoid
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short
# Benchmarking PyTorch: sigmoid
iters: 200, 462.6029555220157
iters: 400, 441.04792759753764
iters: 800, 441.81562116136774
iters: 1600, 440.79964311094955
iters: 3200, 436.3108493271284
iters: 6400, 440.87966314691585
iters: 12800, 452.29464218209614
# Mode: Eager
# Name: sigmoid_M512_N512
# Input: M: 512, N: 512
Forward Execution Time (us) : 441.048
Reviewed By: hl475
Differential Revision: D18149525
fbshipit-source-id: 5fe70a35b790ee7ad3ff57c0cb0b1c29cb609b83
