[FRONTEND] Add an assert in case we get a CPU tensor. (#478)

python/triton/code_gen.py

@@ -689,6 +689,10 @@ class Kernel:
         # handle annotations
         for pos, _type in self.fn.annotations.items():
             wargs[pos] = _type(wargs[pos])
+        # check that tensors are on GPU.
+        for arg in wargs:
+            if hasattr(arg, 'data_ptr'):
+                assert arg.is_cuda, "All tensors must be on GPU!"
         # query device index and cuda stream
         device = torch.cuda.current_device()
         torch.cuda.set_device(device)