Introduce hydra framework with backwards compatibility

[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]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/11027

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As of commit e72c3d8 with merge base d67fb52 ():
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[github-actions]

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