Fix torchchat model not working on demo apps (#3668)

Summary:
As titled fix https://github.com/pytorch/torchchat/issues/810

Pull Request resolved: https://github.com/pytorch/executorch/pull/3668

Test Plan: Use torchchat to export llama3 pte file, run it in executorch, see it running

Reviewed By: kirklandsign

Differential Revision: D57678514

Pulled By: larryliu0820

fbshipit-source-id: 73437a8b2e4aeeb562e3b1bd53d0fe1411d81456