Convert args to LlmConfig

[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: D75263990

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/11081

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✅ You can merge normally! (2 Unrelated Failures)

As of commit 65aa3b0 with merge base f24094e ():

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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