Allow building cuda extension without a device. (#5535)

Added FORCE_CUDA environment variable support, to enable building extensions where a GPU device is not present but cuda libraries are.

extensions/cuda_extension.py

@@ -28,7 +28,9 @@ class _CudaExtension(_CppExtension):
         try:
             import torch
 
-            cuda_available = torch.cuda.is_available()
+            # torch.cuda.is_available requires a device to exist, allow building with cuda extension on build nodes without a device
+            # but where cuda is actually available.
+            cuda_available = torch.cuda.is_available() or bool(os.environ.get('FORCE_CUDA', 0))
         except:
             cuda_available = False
         return cuda_available