[PyTorch] Optimize no input NVTX collection (#70133)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70133
we were creating `sstream` + string concats via `getNvtxStr` even when there were no inputs and wasting precious time. this diff avoids `stringstream` when there is no input to squeeze performance. 60% reduction in overhead
Test Plan:
Before
```
I1214 22:48:07.964118 2971180 bench.cpp:154] Mean 0.970494
I1214 22:48:07.964139 2971180 bench.cpp:155] Median 0.969054
I1214 22:48:07.964144 2971180 bench.cpp:156] Min 0.962247
I1214 22:48:07.964148 2971180 bench.cpp:157] stddev 0.00774841
I1214 22:48:07.964154 2971180 bench.cpp:158] stddev / mean 0.00798398
```
After
```
I1214 22:59:00.039872 3437853 bench.cpp:154] Mean 0.384333
I1214 22:59:00.039896 3437853 bench.cpp:155] Median 0.384886
I1214 22:59:00.039899 3437853 bench.cpp:156] Min 0.370235
I1214 22:59:00.039902 3437853 bench.cpp:157] stddev 0.00435907
I1214 22:59:00.039907 3437853 bench.cpp:158] stddev / mean 0.0113419
```
Reviewed By: aaronenyeshi, robieta
Differential Revision: D33137501
fbshipit-source-id: ce0e8cf9aef7ea22fd8aed927e76be4ca375efc3
