report per iteration execution time (#27923)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27923
As title
Test Plan:
```
buck run caffe2/benchmarks/operator_benchmark/pt:add_test -- --iterations 3 --ai_pep_format true
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short
# Benchmarking PyTorch: add
PyTorchObserver {"type": "add_M64_N64_K64_cpu", "metric": "latency", "unit": "us", "value": "0.027768373489379883"}
PyTorchObserver {"type": "add_M64_N64_K64_cpu", "metric": "latency", "unit": "us", "value": "0.02661752700805664"}
PyTorchObserver {"type": "add_M64_N64_K64_cpu", "metric": "latency", "unit": "us", "value": "0.026746749877929688"}
...
Reviewed By: hl475
Differential Revision: D17911718
fbshipit-source-id: 6fe28f2ab9ce1e0feabb5b822f04ff32dac977a9
