Add new export LLM config

[jackzhxng]

Stack from ghstack (oldest at bottom):

• Introduce hydra framework with backwards compatibility #11027
• -> Add new export LLM config #11026
• Add new export LLM config #11025

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/11026

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As of commit 96816a2 with merge base d67fb52 ():
💚 Looks good so far! There are no failures yet. 💚

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[github-actions]

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