Use a reference in GetSingleArgument (#71007)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71007
A string copy at Line 417 is currently consuming 125,749,287,000 cycles/day. I suspect the issue is with a copy-on-return, but we can experiment with introducing a reference in the middle to see if that produces a good savings without changing the interface.
Reference
```
["Inline caffe2::ArgumentHelper::GetSingleArgument @ caffe2/caffe2/utils/proto_utils.cc:417"]
```
Test Plan: Sandcastle
Reviewed By: xw285cornell
Differential Revision: D33478883
fbshipit-source-id: e863e359c0c718fcd0d52fd4b3c7858067de0670
