[codex] Preserve Arm partition tag order

[Praneeth1636]

Summary

• preserve Arm partition discovery order in TOSAPartitioner instead of materializing tags through an unordered set
• keep partition() emitting partition_tags in that stable discovery order
• add focused regression tests for tag ordering in the Arm partitioner

Why

• fixes nondeterministic Arm export/lowering order when Python hash randomization changes set iteration order
• keeps backend artifacts stable across fresh processes for the same model and weights

Fixes #19045

Testing

• PYTHONPATH=/tmp/executorch-test-stubs:src .venv/bin/python -m pytest --config-file=/dev/null backends/arm/test/misc/test_partitioner_tag_order.py

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell

[pytorch-bot]

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❌ 10 Awaiting Approval, 11 New Failures, 22 Pending, 1 Unrelated Failure

As of commit f2e5c7c with merge base c48ea12 ():

AWAITING APPROVAL - The following workflows need approval before CI can run:

• Build documentation (gh)
• Build Presets (gh)
• Cadence Build & Test (gh)
• Lint (gh)
• periodic (gh)
• Test ARM Backend (gh)
• Test CoreML Backend (gh)
• Test QNN Backend (gh)
• Test Vulkan Backend (gh)
• Test XNNPACK Backend (gh)

NEW FAILURES - The following jobs have failed:

• trunk / test-models-macos-coreml (emformer_join) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos-coreml (emformer_transcribe) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos-coreml (resnet50) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos-cpu (efficient_sam, portable) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos-cpu (ic4, xnnpack-quantization-delegation) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos-cpu (llama, portable) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos-cpu (llama2, xnnpack-quantization-delegation) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos-cpu (mobilebert, xnnpack-quantization-delegation) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos-cpu (w2l, xnnpack-quantization-delegation) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos-mps / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-selective-build-macos (cmake) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-models-linux (dl3) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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[perheld]

I dont think this is needed here if you are not a Arm employee. Once/if we touch the file we will add the copyright.