Add USE_CUDNN check to AT_CUDNN_ENABLED definition (#25037)
Summary:
We have environment variable USE_CUDNN with self-explanatory name. However cpp code is compiled based on cpp macro definition AT_CUDNN_ENABLED, which is defined as:
```
IF (NOT AT_CUDA_ENABLED OR NOT CUDNN_FOUND)
MESSAGE(STATUS "CuDNN not found. Compiling without CuDNN support")
set(AT_CUDNN_ENABLED 0)
ELSE()
include_directories(SYSTEM ${CUDNN_INCLUDE_DIRS})
set(AT_CUDNN_ENABLED 1)
ENDIF()
```
So, even if USE_CUDNN is set to 0, cpp is compiled with cuDNN if cmake finds cuDNN in the system. I actually tested it and was very surprised when I was debugging cuDNN code which I built with USE_CUDNN=0. I believe that cmake code above should look like this:
`IF (NOT AT_CUDA_ENABLED OR NOT CUDNN_FOUND OR NOT USE_CUDNN) ...`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25037
Differential Revision: D17048683
Pulled By: pbelevich
fbshipit-source-id: 48afa19eaae0bba2ffd49c1f68db0b4efd5cf85e
