[Draft] iOS support

[metascroy]

@seyeong-han

Here is a draft PR for frameworks:

scripts/build_apple_frameworks.sh --mlx --ios-only

Once built, you need to link the etLLM demo app with them.

I have not been able to get gemma3-4b to run with vision without iOS killing the app due to memory pressure. I also notice some perf issues in the CPU sampler inside etLLM (around 25% of the time is spent in the sampler), which I think is related to lack of BF16 simd on iOS.

All this is to say that things on iOS are very rough.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17991

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❌ 8 New Failures, 2 Unrelated Failures

As of commit f79d8fa with merge base 75f5a76 ():

NEW FAILURES - The following jobs have failed:

• MLX / backend-tester (models) / test-mlx-backend-models (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• MLX / test-mlx-llm (unsloth/gemma-3-1b-it, gemma3-1b, false, 4w) / test-mlx-llm-gemma3-1b-4w (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• MLX / test-mlx-llm (unsloth/gemma-3-1b-it, gemma3-1b, false, nvfp4) / test-mlx-llm-gemma3-1b-nvfp4 (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• MLX / test-mlx-voxtral-realtime / test-mlx-voxtral-realtime (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 127
• Test Metal Backend / export-model-metal-artifact (nvidia, parakeet-tdt, non-quantized) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (openai, whisper-large-v3-turbo, quantized-int4-metal) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (openai, whisper-small, non-quantized) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (openai, whisper-small, quantized-int4-metal) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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 / macos / macos-job (gh) (trunk failure)
  AttributeError: '_OpNamespace' 'mkldnn' object has no attribute '_is_mkldnn_acl_supported'
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  AttributeError: '_OpNamespace' 'mkldnn' object has no attribute '_is_mkldnn_acl_supported'

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