Update dynamo cpu fallback op to aten::_foobar (#5393)

Author: wonjoo-wj

test/dynamo/test_dynamo.py

@@ -175,8 +175,8 @@ class DynamoCpuFallbackTest(unittest.TestCase):
  def test_operator_fallback(self):
 
  def fn_fallback(t):
- # As of 05/18/2023, torch.median is not lowered by PyTorch/XLA
- return torch.median(t)
+ # aten::_foobar is aux function that's used for testing purposes only
+ return torch._foobar(t)
 
  torch._dynamo.reset()
  met.clear_counters()
@@ -211,8 +211,8 @@ def test_fallback_multiple_submodules(self):
 
  def fn_fallback(t):
  t_2 = torch.mul(t, 2)
- # As of 05/18/2023, torch.median is not lowered by PyTorch/XLA
- t_3 = torch.median(t_2)
+ # aten::_foobar is aux function that's used for testing purposes only
+ t_3 = torch._foobar(t_2)
  t_4 = torch.mul(t_3, 2)
  return t_4