Move LM wrapper generation into helper (#2609)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2609

Part of a refactor of eval_llama_lib to support integration with runtime evaluation and removing model quantization from the eval flow.

Specifically this diff just moves the Wrapper generation logic into a helper

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Reviewed By: iseeyuan

Differential Revision: D55273853

fbshipit-source-id: 98e1e6108454f39d90aa79cc09024d732254aa64