Move the detection of cuDNN to FindCUDNN.cmake (#24784)
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`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24784
Differential Revision: D16914345
Pulled By: ezyang
fbshipit-source-id: fd261478c01d879dc770c1f1a56b17cc1a587be2
