Export LLM documentation revamp

[jackzhxng]

Summary

Structure:

• New Getting Started page
• AOT (export)
  • Old getting started page, which was the NanoGPT tutorial, is moved to export-custom-llm.md, with the runner sections removed to add to the run-with-c-plus-plus.md
  • New export-llm.md page for exporting LLMs with export_llm API
• Runtime
  • iOS/Android app docs remain, they detail steps to take after the .pte is generated for running on-device
  • Added a C++ runner page for @larryliu0820 to fill out with the new runner APIs
  • Since the QNN Llama tutorial is highly custom, we are going to leave the export section in it as well instead of dividing like we did for the rest of the tutorials

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12381

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure

As of commit 6af121b with merge base dd4488d ():

NEW FAILURE - The following job has failed:

• pull / test-eval_llama-mmlu-linux / linux-job (gh)
  RuntimeError: Command docker exec -t ac7f1dfa05198559187d30f3fc37831340c39c6bf811e9e175c1d22cb0e5d6a9 /exec failed with exit code 1

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[github-actions]

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[mergennachin]

Why is this page called "Exporting"? Shouldn't this be high-level than just export? Presumably, you'll talk about runtime etc.

[mergennachin]

How about just "Deploying LLM using ExecuTorch"?