[dynamo] Guard serialization for DUPLICATE_INPUT. (#152687)

Seems this guard is not very active. Adding a test to detect error handling at least.

Differential Revision: [D74074837](https://our.internmc.facebook.com/intern/diff/D74074837/)

Pull Request resolved: https://github.com/pytorch/pytorch/pull/152687
Approved by: https://github.com/jansel
ghstack dependencies: #152615, #152616

test/dynamo/test_guard_serialization.py

@@ -446,6 +446,16 @@ class TestGuardSerialization(torch._inductor.test_case.TestCase):
         with torch._functorch.eager_transforms.grad_increment_nesting():
             self._test_check_fn(ref, loaded, {"x": x}, False)
 
+    def test_duplicate_input(self):
+        def fn(x, x_):
+            return x + x_
+
+        x = torch.randn(3, 2)
+        with self.assertRaisesRegex(
+            RuntimeError, "DUPLICATE_INPUT guard cannot be serialized"
+        ):
+            self._test_serialization("DUPLICATE_INPUT", fn, x, x)
+
 
 if __name__ == "__main__":
     from torch._dynamo.test_case import run_tests

torch/_dynamo/guards.py

@@ -1838,6 +1838,8 @@ class GuardBuilder(GuardBuilderBase):
 
     # TODO(voz): Deduplicate w/ AOTAutograd dupe input guards
     def DUPLICATE_INPUT(self, guard, source_b):
+        if self.serialization_mode == "save":
+            raise RuntimeError("DUPLICATE_INPUT guard cannot be serialized yet.")
         ref_a = self.arg_ref(guard)
         ref_b = self.arg_ref(source_b.name())