Build GPU extensions whenever CUDA_HOME is available (#911)

This is the same logic and motivation as in
https://github.com/pytorch/pytorch/pull/8244/
that is to build a docker image (which has no GPU available at build time),
and then use it with nvidia-docker

setup.py

@@ -88,7 +88,7 @@ def get_extensions():
 
     define_macros = []
 
-    if torch.cuda.is_available() and CUDA_HOME is not None:
+    if CUDA_HOME is not None:
         extension = CUDAExtension
         sources += source_cuda
         define_macros += [('WITH_CUDA', None)]