[export] Copy gm before calling PassManager (pytorch#108321)

Test Plan: CI

Reviewed By: angelayi, cccclai

Differential Revision: D48801487

Pull Request resolved: pytorch#108321
Approved by: https://github.com/kimishpatel, https://github.com/mcr229

torch/export/__init__.py

@@ -398,7 +398,7 @@ def _transform(self, *passes: PassType) -> "ExportedProgram":
         )
 
         pm = PassManager(list(passes))
-        res = pm(self.graph_module)
+        res = pm(copy.deepcopy(self.graph_module))
         transformed_gm = res.graph_module if res is not None else self.graph_module
         assert transformed_gm is not None