skip loading of upfirn2d to run without CUDA

sgmse/backbones/ncsnpp_utils/op/__init__.py

-from .fused_act import FusedLeakyReLU, fused_leaky_relu
 from .upfirdn2d import upfirdn2d
\ No newline at end of file

sgmse/backbones/ncsnpp_utils/op/upfirdn2d.py

@@ -7,6 +7,8 @@ from torch.utils.cpp_extension import load
 
 
 module_path = os.path.dirname(__file__)
+
+if torch.cuda.is_available():
     upfirdn2d_op = load(
         "upfirdn2d",
         sources=[
@@ -14,7 +16,8 @@ upfirdn2d_op = load(
             os.path.join(module_path, "upfirdn2d_kernel.cu"),
         ],
     )
-
+else:
+    upfirdn2d_op = None
 
 class UpFirDn2dBackward(Function):
     @staticmethod