Work around spec violation in executorch program unit test. (#453)

Summary:
Pull Request resolved: #453

In theory, we shouldn't serialize an executorch program with default serializer because there is a spec violation with out variant graph.

Reviewed By: angelayi

Differential Revision: D49548576

fbshipit-source-id: cedf9810c594b33019eb50514e645c36289ce192

Author: zhxchen17

exir/tests/test_serde.py

@@ -56,7 +56,8 @@ def check_serde(self, m, inputs) -> None:
 
         executorch = edge.to_executorch().dump_exported_program()
         executorch_new = deserialize(*serialize(executorch))
-        self.check_ep(executorch, executorch_new, inputs)
+        with torch.no_grad():
+            self.check_ep(executorch, executorch_new, inputs)
 
     def test_basic(self) -> None:
         class MyModule(torch.nn.Module):