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
• [ET][Metal] Update aoti_common with additional AOTI functions needed by Metal backend #15003

[pytorch-bot]

🔗 Helpful Links

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

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❌ 3 New Failures

As of commit 1a22c5e with merge base 896178e ():

NEW FAILURES - The following jobs have failed:

• pull / test-qnn-wheel-packages-linux (3.10) / linux-job (gh)
  RuntimeError: Command docker exec -t 5207ee1926e9c22704a92a900e5662c307929cd366497d1045bdc340ecd78d77 /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.11) / linux-job (gh)
  RuntimeError: Command docker exec -t c21a00d55d6ac7f7c139735e40e887619a424ff22ea0d562176348ed6e20168f /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.12) / linux-job (gh)
  RuntimeError: Command docker exec -t d770a9f78f49e4b227dd996a64dfcf480d4dd2381aea30926bc0269e5ce26a27 /exec failed with exit code 1

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