Arm backend: Add docstrings for tosa/partitioner.py

[Sebastian-Larsson]

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14844

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❌ 3 New Failures, 1 Unrelated Failure

As of commit b7eb0a6 with merge base 0b748bf ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 7bdde6d46b0985cb0f2290e2c4f7c9146fdcc81c41e681e55f6e50180e8a32b6 /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  examples/models/test/test_export.py::ExportTest::test_ic3_export_to_executorch
• trunk / unittest-release / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_linear_model

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Apple / build-demo-ios / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 74

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