Add Metal backend Python preprocessing, partitioning, and tests

[manuelcandales]

Implement Python-side Metal backend interface including:

• MetalBackend: AOT compilation with MPS device support
• MetalPartitioner: Graph partitioning for Metal delegation

Add unit tests for partitioner and backend preprocessing.

[manuelcandales]

Stack from ghstack (oldest at bottom):

• Add Metal backend build system and runtime integration #15024
• Metal backend: Add operator implementations #15023
• Metal backend: Implement the AOTI MPS shim #15022
• Metal backend: Add AOTI shims for memory management #15021
• Add Metal backend core ETMetal runtime. #15020
• Add Metal backend type definitions and utilities #15019
• -> Add Metal backend Python preprocessing, partitioning, and tests #15015

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15015

• 📄 Preview Python docs built from this PR

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❗ 1 Active SEVs

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• [ROCm][CI] Machines under the label linux.rocm.gpu.2 are undergoing maintenance.

❌ 5 New Failures, 7 Unrelated Failures

As of commit 89d3f14 with merge base 9560800 ():

NEW FAILURES - The following jobs have failed:

• Apple / build-demo-ios / macos-job (gh)
• pull / test-qnn-wheel-packages-linux (3.10) / linux-job (gh)
  RuntimeError: Command docker exec -t f040026ed99eac5edea0301f8f432abfec57cf3c27ec5c061fb1b8c7f116ef33 /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.11) / linux-job (gh)
  RuntimeError: Command docker exec -t 73c129c5291a9e9dc74987f74d436f7d8e720f6571e392b8983334d632f0ff96 /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.12) / linux-job (gh)
  RuntimeError: Command docker exec -t 94c3c235c6ace8c035a8c0a225287f52ef9aa18e07cd6e05f51dae646d73a741 /exec failed with exit code 1
• Test CUDA Builds / export-voxtral-cuda-artifact / linux-job (gh)
  RuntimeError: Command docker exec -t 4f38b66430a85a5486979fce777d1c009dfe704b62018318a1c9d8274a6efce5 /exec failed with exit code 1

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

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

• Apple / build-benchmark-app / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Apple / build-frameworks-ios / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• pull / unittest / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / unittest-buck / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[mergennachin]

Add a readme in backends/apple/metal/README.md

Say that this is an experimental backend