[nnc] Don't fuse fp16 on CPU (#56119)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56119
There are apparently still more issues with fp16 on LLVM so let's just
nuke it from orbit while we develop a robust workaround.
ghstack-source-id: 126619411
Test Plan: compile
Reviewed By: ZolotukhinM
Differential Revision: D27787080
fbshipit-source-id: 9e771211fe48266f50fca1de8d40295922da5bca
