accept -1 in iterations and warmup iterations (#28014)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28014
as title
Test Plan:
```
buck run caffe2/benchmarks/operator_benchmark/pt:add_test -- --iterations -1 --warmup_iterations -1
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short
# Benchmarking PyTorch: add
# Mode: Eager
# Name: add_M64_N64_K64_cpu
# Input: M: 64, N: 64, K: 64, device: cpu
Forward Execution Time (us) : 30827.046
...
Reviewed By: hl475
Differential Revision: D17932071
fbshipit-source-id: e4d9d256a0a4958110f61af13afdde70fc0f746c
