Improve static typing in tracer. (#3973)

Summary:
Pull Request resolved: #3973

X-link: pytorch/pytorch#128552

as title.

Reviewed By: angelayi

Differential Revision: D58485487

fbshipit-source-id: b2ae3f09e9465fb7f05a49d5a82f491dce5c6fe0

Author: zhxchen17

exir/lowered_backend_module.py

@@ -24,6 +24,7 @@
 from executorch.exir.schema import Program
 
 from executorch.exir.tracer import Value
+from torch._library.fake_class_registry import FakeScriptObject
 
 from torch._subclasses import FakeTensor
 from torch.export.exported_program import (
@@ -430,7 +431,7 @@ def _get_new_signature(  # noqa: C901
 ) -> Tuple[
     ExportGraphSignature,
     Dict[str, Union[torch.Tensor, torch.nn.Parameter]],
-    Dict[str, Union[torch.Tensor, torch.ScriptObject]],
+    Dict[str, Union[torch.Tensor, FakeScriptObject, torch.ScriptObject]],
 ]:
     """
     Args: