Fix pt2e test_qat_preserve_source_fn_stack (#3149)

**Summary:** A recent change in `torch.export` now inlines the
submodules in the `source_fn_stack`. We should adjust our test
accordingly to unbreak CI.

**Test Plan:**
python test/quantization/pt2e/test_quantize_pt2e_qat.py -k test_qat_preserve_source_fn_stack

Author: andrewor14

test/quantization/pt2e/test_quantize_pt2e_qat.py

@@ -686,12 +686,6 @@ def get_source_fn(node: torch.fx.Node):
         self.assertNotEqual(get_source_fn(second_conv), get_source_fn(second_relu))
         self.assertNotEqual(get_source_fn(first_relu), get_source_fn(second_relu))
 
-        # Assert that "backbone" exists only in the second set of conv and relu's partition
-        self.assertTrue("backbone" not in get_source_fn(first_conv))
-        self.assertTrue("backbone" not in get_source_fn(first_relu))
-        self.assertTrue("backbone" in get_source_fn(second_conv))
-        self.assertTrue("backbone" in get_source_fn(second_relu))
-
     def test_qat_conv_bn_bias_derived_qspec(self):
         m = self._get_conv_bn_model()
         example_inputs = self.example_inputs