ExecuTorch MLX delegate

[metascroy]

Summary

This PR adds an MLX backend for ExecuTorch, enabling Metal-accelerated inference on Apple Silicon. It runs Llama, Qwen, Gemma, Whisper, Voxtral, and Parakeet models end-to-end, with 637 passing op tests and multithreaded execution support. For many models, it offers best performance among all ExecuTorch backends on Apple Silicon, offering 2-6x speedups over what was previously possible with ExecuTorch, and up to 30% smaller model sizes compared to XNNPACK due to BF16 support and tied quantized embedding support.

The PR is large due to extensive op coverage, testing, and documentation, but almost all changes are confined to backends/mlx/. The design is described in backends/mlx/README.md.

Suggested review approach:

1. Review files outside backends/mlx/ carefully — these integrate with ExecuTorch's build system and are the most likely to need changes.
2. For backends/mlx/, focus on structural design (see README) and test coverage (CI job is .github/workflows/mlx.yml)

Prerequisite PRs

These fixes were developed alongside the MLX backend. Once merged, this PR can be rebased to remove the duplicated changes:

• #17257 — Improve lowering time with NamedDataMap
• #17679 — Allow transform passes in etLLM
• #17678 — Fix dynamic shape bug in remove_noop_pass
• #17378 — Fix pocketfft intermittent bus errors on macOS (upstream fix)

Tests

CI is defined in .github/workflows/mlx.yml:

• test_ops.py: 637 passing op tests
• Multithreading: launches models on 50 threads, verifies correctness
• GenAI E2E: parakeet, voxtral, etLLM (stories110m), HF LLMs (llama, qwen, gemma)
• backend-tester: 380 passed, 1 failed, 86 skipped operator tests; 34 passed, 3 failed, 5 skipped model tests

The 1 failing operator test is a test-side issue being fixed in #17539. The 3 failing model tests will be addressed in follow-ups — they are not an initial focus compared to the GenAI models above.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/16718

• 📄 Preview Python docs built from this PR

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

❌ 2 Cancelled Jobs

As of commit 240b241 with merge base 3319157 ():

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• Test ARM Backend / test-arm / test-backend-linux (arm_tosa_fp, models) / linux-job (gh)
  ##[error]The operation was canceled.
• Test ARM Backend / test-arm / test-backend-linux (arm_vgf_fp, models) / linux-job (gh)
  ##[error]The operation was canceled.

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