Fix -Wundef warning in conversions.h
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/31911
Test Plan:
* CI builds including GPU and OSS-build tests
* The `defined(__HIP_DEVICE_COMPILE__) ` instance a few lines below is proof that this is a define/undef flag, not a define01 flag
Reviewed By: hlu1
Differential Revision: D19296560
fbshipit-source-id: 1c45069aec534b0bf4a87751a74680675c985e06
