[PyTorch] Add C10_ prefix to MPARK_* macros in variant.h (#65589)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65589
Without this prefix, the include guards interfere with attempts to indirectly include both c10::variant and the original mpark variant in the same translation unit.
ghstack-source-id: 138901838
Test Plan: Temporarily `#include <c10/util/variant.h>` in ivalue.h and buck build //data_preproc/preproc:preproc_adapter_utils mode/no-gpu -- this delayed D31101962 (https://github.com/pytorch/pytorch/commit/01720d6a2352fc1c16f41753c3929f9c12dac528) from fixing S244170
Reviewed By: bhosmer
Differential Revision: D31159414
fbshipit-source-id: 234c5ed37ca853702bcdf3263e4f185b95ac1d08
