Remove redundant serialization code (#126249)

After #123308, we no longer need separate serialization path to handle different types that exist in the `nn_module` metadata. This PR cleans up the redundant code.
Pull Request resolved: #126249
Approved by: https://github.com/angelayi

torch/_export/serde/serialize.py

@@ -539,21 +539,9 @@ def export_nn_module_stack(val):
                 path, ty = val
 
                 assert isinstance(path, str)
+                assert isinstance(ty, str)
 
-                # node.meta["nn_module_stack"] could have two forms:
-                # 1. (path: str, module_type: 'type'), e.g.
-                #    ('', <class 'sigmoid.inference.MySimpleModel'>)
-                # 2. (path: str, module_type: str), e.g.
-                #    ('', 'sigmoid.inference.MySimpleModel')
-                # ExportedProgram directly produced by torch.export() has form 1
-                # ExportedProgram deserialized from disk has form 2
-                # TODO: This is not ideal, we should fix this.
-                if isinstance(ty, str):
-                    normalized_ty = ty
-                else:
-                    normalized_ty = ty.__module__ + "." + ty.__qualname__
-
-                return path + "," + normalized_ty
+                return path + "," + ty
 
             # Serialize to "key,orig_path,type_str"
             nn_module_list = [