Fix Wextra issues in Half.h (#66643)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66643
Fixes:
```
caffe2/c10/util/Half.h:456:14: error: comparison of integers of different signs: 'long' and 'unsigned long' [-Werror,-Wsign-compare]
return f > limit::max() ||
~ ^ ~~~~~~~~~~~~
```
Test Plan: Sandcastle
Reviewed By: ngimel
Differential Revision: D31656816
fbshipit-source-id: 7623d20e166a9e95a949ebd8b23793f24960cf07
