add back "CPU" in test_onnx_backend supports_device (#3426)

the onnx tests were all skipped.

test/test_tensor.py

@@ -414,7 +414,6 @@ class TestZeroShapeTensor(unittest.TestCase):
     assert t.shape == (3, 2, 2)
     np.testing.assert_equal(t.numpy(), s.numpy())
 
-    # torch does not support padding non-zero dim with 0-size. torch.nn.functional.pad(torch.zeros(3,2,0), [0,0,0,4,0,0])
     s = Tensor.rand(3, 4, 0)
     t = Tensor.rand(3, 2, 0).cat(s, dim=1)
     assert t.shape == (3, 6, 0)