fix viewable return

Author: leofang

cuda_py/cuda/py/_memoryview.pyx

@@ -275,6 +275,6 @@ def viewable(tuple arg_indices):
             cdef int idx
             for idx in arg_indices:
                 args[idx] = _GPUMemoryViewProxy(args[idx])
-            func(*args, **kwargs)
+            return func(*args, **kwargs)
         return wrapped_func
     return wrapped_func_with_indices