Fix build with cuda but no cudnn in caffe2 (#16701)
Summary:
Just noticed while building on a machine without cudnn present - it was building but the runtime failed since some methods weren't bound
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16701
Differential Revision: D13937247
Pulled By: dzhulgakov
fbshipit-source-id: c81f05be7a9e64a1a8591036dcf8692c0ed4064e
