Introduce hydra framework with backwards compatibility

[jackzhxng]

Stack from ghstack (oldest at bottom):

• Use llm_config instead of args in export_llama functions #11084
• Convert args to LlmConfig #11081
• -> Introduce hydra framework with backwards compatibility #11029
• Add new export LLM config #11028

Differential Revision: D75263989

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/11029

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❌ 1 New Failure, 2 Unrelated Failures

As of commit 6c6cf65 with merge base f24094e ():

NEW FAILURE - The following job has failed:

• pull / test-llava-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 1898794c49ac25ea015c7ff4ad2aba716b2d0feed7a3ba19f5271ff43061ecba /exec failed with exit code 139

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / linux / linux-job (gh) (trunk failure)
  examples/models/llama/tests/test_export_llama_lib.py::ExportLlamaLibTest::test_has_expected_ops_and_op_counts
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
  examples/models/llama/tests/test_export_llama_lib.py::ExportLlamaLibTest::test_has_expected_ops_and_op_counts

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[jackzhxng]

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