Remove setter for graph_module

Summary:
X-link: pytorch/pytorch#106651

The ExportedProgram should be immutable

Reviewed By: zhxchen17

Differential Revision: D48086375

fbshipit-source-id: c5da030f0fd9ed4fa78449161e9d6e1b57b32645

Author: angelayi

backends/xnnpack/test/tester/tester.py

@@ -152,7 +152,7 @@ def run(
     ) -> None:
         prepared = prepare_pt2e(artifact.exported_program.graph_module, self.quantizer)
         converted = convert_pt2e(prepared)
-        artifact.exported_program.graph_module = converted
+        artifact.exported_program._graph_module = converted
         self.converted_program = artifact
 
     @property

exir/backend/backend_api.py

@@ -287,8 +287,16 @@ def to_backend(
         tagged_graph_module, partitioner_instance, edge_program
     )
 
-    edge_program.graph_module = tagged_graph_module
-    return edge_program
+    return ExportedProgram(
+        tagged_graph_module,
+        tagged_graph_module.graph,
+        copy.deepcopy(edge_program.graph_signature),
+        copy.deepcopy(edge_program.call_spec),
+        edge_program.state_dict,
+        copy.deepcopy(edge_program.range_constraints),
+        copy.deepcopy(edge_program.equality_constraints),
+        copy.deepcopy(edge_program.module_call_graph),
+    )
 
 
 def to_backend_multiple(