fix a set union bug (#2827)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2827

It's a bug, we need to keep union the original set with the new set from the for loop, however the original code just overwrite the original set. Test with coreml + llama code

```
python3 -m examples.models.llama2.export_llama --coreml --use_kv_cache
```

Reviewed By: angelayi

Differential Revision: D55680296

fbshipit-source-id: 906d508a276a91bd4b1caa295ce547e9686612ad