Revert "Build GPU extensions whenever CUDA_HOME is available (#911)" (#924)

This reverts commit b384c4e7.

setup.py

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