[MLX] C++ runner for HF LLMs with the off-graph KV cache

[kiymetakdemir]

Summary

This runner builds an MLXSequenceCache, installs it, and passes the cache key, so it's the run path for .pte files exported with --use-offgraph-cache. The cache's shape is read from the .pte metadata. The flags left are policy the model can't imply: --kv-max-capacity, --kv-storage-dtype, --kv-initial-capacity, --kv-max-write, --kv-windows.

Depends on #21680; the new CI job fails until that lands.

Files

• run_llm_hf.cpp — the runner: chat templates, greedy decode, benchmarking, and an interactive mode with /reset and /undo [N].
• CMakeLists.txt — standalone find_package(executorch) project.
• .github/workflows/mlx.yml — test-mlx-llm-offgraph for llama-1b, gemma3-1b and gemma4-e2b.

Test
CI builds the runner, then for llama-1b, gemma3-1b and gemma4-e2b exports off-graph and asserts the same "Paris" answer test-mlx-llm checks.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21683

• 📄 Preview Python docs built from this PR

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✅ No Failures

As of commit fed3f79 with merge base 14326d9 ():
💚 Looks good so far! There are no failures yet. 💚

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