Move the detection of cuDNN to FindCUDNN.cmake (#24938)
Summary:
Currently they sit together with other code in cuda.cmake. This commit is the first step toward cleaning up cuDNN detection in our build system.
Another attempt to https://github.com/pytorch/pytorch/issues/24293, which breaks manywheels build because it does not handle `USE_STATIC_CUDNN` properly.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24938
Differential Revision: D17070920
Pulled By: ezyang
fbshipit-source-id: a4d017a3505c102d9c435a73ae62332e4336c52e
