[cuDNN][cuDNN V8 API] Always build assuming cuDNN >= 8.0 (#91527)
We've been building with V8 (incl. V8 API) by default for a while now; this PR cleans up some guards for cuDNN < 8.0.
CC @ptrblck @ngimel
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91527
Approved by: https://github.com/ngimel
