code: def test_undefined_inputs_outputs(self, mode):
      with autograd_fallback_mode(mode):
          lib = self.get_lib()
          lib.define("foo(Tensor a, Tensor b) -> (Tensor, Tensor)")
          op = self.get_op("foo")
  
          def foo_impl(a, b):
              return None, b.clone()
  
          lib.impl("foo", foo_impl, "CPU")
  
          x = torch.randn(3, requires_grad=True)
          # NB: PyTorch dispatcher treats "None" as undefined Tensor.
          y, z = op(None, x)
          with self._check_ctx(mode):
              z.sum().backward()
  
imports: import contextlib
  import warnings
  import numpy
  import torch
  from torch.library import Library
  from torch.testing._internal.common_utils import instantiate_parametrized_tests, parametrize, run_tests, TestCase
label_desc: Use image node_id 14 for calling the TestAutogradFallback obj's underlying member method code with example usage: obj.test_undefined_inputs_outputs(mode) and returns: None,  b