Fix output ordering in lowered modules to prevent spec violation error

[metascroy]

When the ExecuTorch partitioner creates submodules from a partitioned graph, _get_new_signature builds output specs by iterating the submodule's output tuple in graph order. This can produce output specs where BUFFER_MUTATION and USER_OUTPUT kinds are interleaved. The ExportedProgram verifier expects all mutation outputs to precede user outputs ([buffer_mutations..., user_outputs...]), and a misordered tuple causes a SpecViolationError.

This PR adds arrange_graph_outputs, which reorders the submodule's output tuple and output specs so that all mutation outputs (BUFFER_MUTATION, USER_INPUT_MUTATION) come before USER_OUTPUT outputs. It also remaps getitem indices on the parent graph's call_module node so the parent continues to extract the correct values after reordering.

Changes
exir/lowered_backend_module.py

Added arrange_graph_outputs() function that:

• Reorders the submodule graph's output node to put mutation outputs first
• Reorders output_specs in-place to match
• Remaps getitem indices in the parent graph

Test plan:

Existing CI + new unit test

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17750

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure, 1 Pending, 2 Unrelated Failures

As of commit 6e2fd3b with merge base 1f4ad07 ():

NEW FAILURE - The following job has failed:

• Test Metal Backend / test-model-metal-e2e (mistralai, Voxtral-Mini-4B-Realtime-2602, 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

• Test CUDA Windows Export and E2E / test-model-cuda-windows-e2e (mistralai, Voxtral-Mini-3B-2507, non-quantized) / windows-job (gh) (trunk failure)
  Process completed with exit code 1.
• Test CUDA Windows Export and E2E / test-model-cuda-windows-e2e (mistralai, Voxtral-Mini-3B-2507, quantized-int4-weight-only) / windows-job (gh) (trunk failure)
  Process completed with exit code 1.

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

[github-actions]

This PR needs a release notes: label

If your change should be included in the release notes (i.e. would users of this library care about this change?), please use a label starting with release notes:. This helps us keep track and include your important work in the next release notes.

To add a label, you can comment to pytorchbot, for example
@pytorchbot label "release notes: none"

For more information, see
https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.