Don't create FusionGroups for known-CPU producer values (#19342)
Summary:
I believe the existing check in FuseGraph was only `false` if PyTorch was built with NO_CUDA=1. Otherwise, we would create fusion groups even if we're on a CPU-only machine running CPU code. This is confusing. Instead I've made it so that the decision to fuse or not is dependent on if the producer Value is a known CPU tensor. If it is, we skip fusion.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19342
Differential Revision: D15038351
Pulled By: jamesr66a
fbshipit-source-id: fce9d83929309a7bf14346833f84b996f3e7f6db
