Create yaml configs for documented export_llm use cases

[jackzhxng]

Migrate READMEs to use yaml llm configurations for export_llm

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/11938

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⏳ 28 Pending, 5 Unrelated Failures

As of commit 261d9db with merge base 6e706f2 ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-editable / linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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

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

• pull / unittest / linux / linux-job (gh) (trunk failure)
  extension/llm/export/test/test_export_llm.py::TestExportLlm::test_with_config
• pull / unittest / macos / macos-job (gh) (trunk failure)
  extension/llm/export/test/test_export_llm.py::TestExportLlm::test_with_config
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  extension/llm/export/test/test_export_llm.py::TestExportLlm::test_with_config
• trunk / unittest-release / linux / linux-job (gh) (trunk failure)
  extension/llm/export/test/test_export_llm.py::TestExportLlm::test_with_config

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