Skip Bfloat16 support when building for VSX (#61630)
Summary:
Copy-paste ifdef guard from vec256/vec256.h
Probably fixes https://github.com/pytorch/pytorch/issues/61575
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61630
Reviewed By: janeyx99
Differential Revision: D29690676
Pulled By: malfet
fbshipit-source-id: f6d91eadab74bcbcb1dc9854ae1b98a0dccacd14
