Arm backend: Add dim mapping helpers

[AdrianLundell]

Dim args such as sum(dim=1) needs to be transformed when swapping place with operators which change
shape, i.e. permutes and views.

ViewMap and PermuteMap handles and validates these transforms for reduction dims and permute dims.

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

[pytorch-bot]

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❌ 2 New Failures, 22 Pending, 1 Unrelated Failure, 2 Unclassified Failures

As of commit cb08a9e with merge base 7d365ec ():

NEW FAILURES - The following jobs have failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 9235097ae4fb4f361d66455e81d3e8e769866c558d683d254666c0e66a9311ab /exec failed with exit code 139
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t e66a8c3a3b5b49d66dc080ffdbdb557dc5aad0d818759b1c1b660fc3e0b75ff6 /exec failed with exit code 1

UNCLASSIFIED FAILURES - DrCI could not classify the following jobs because the workflow did not run on the merge base. The failures may be pre-existing on trunk or introduced by this PR:

• Cadence Build & Test / vision-build / vision (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Input required and not supplied: aws-region
• trunk / test-models-macos-coreml (resnet50) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  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-arm-backend-no-driver (test_pytest_ops_tosa) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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[Copilot]

Pull request overview

This PR introduces dimension-mapping helpers for the Arm backend to transform reduction/permutation dimension arguments correctly when commuting shape-changing ops (notably view and permute). It also adds a dedicated test suite covering mapping behavior, including symbolic dims.

Changes:

• Add ViewMap to map reduction/permutation dims across view (via grouped prime-factor tracking, with limited SymInt support).
• Add PermuteMap to map dims across permute.
• Add extensive unit tests (including randomized/bruteforce checks and SymInt cases) for the new mapping helpers.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
backends/arm/_passes/dim_maps.py	Implements ViewMap/PermuteMap utilities for mapping dims across view/permute.
backends/arm/test/passes/test_dim_maps.py	Adds tests validating mapping behavior vs. identities and bruteforce enumeration, including SymInt coverage.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

[AdrianLundell]

Arm fails is the unrelated fp8 issue from main.