Arm backend: Enable dynamic shapes in DecomposeMeanDimPass

[SaoirseARM]

Summary

With the upcoming SymInt, enable dynamic shapes in DecomposeMeanDimPass.
Avg_pool2d operator is currently not supported.

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

[pytorch-bot]

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❌ 9 New Failures, 2 Cancelled Jobs, 4 Unrelated Failures

As of commit 8349b47 with merge base 22867a9 ():

NEW FAILURES - The following jobs have failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 7c04f06ac0359c772efe12d608a8eddbcb6c7a9075fde4456ff576f55af279cb /exec failed with exit code 139
• pull / unittest / linux / linux-job (gh)
  examples/models/test/test_export.py::ExportTest::test_efficient_sam_export_to_executorch
• pull / unittest / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model
• pull / unittest-arm-backend-with-no-deps (test_pytest_models_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 22fcf4c1c5aa98ff5383cba17fa04e11a0adf33468e889140b059d39bae66dc0 /exec failed with exit code 1
• trunk / test-huggingface-transformers-xnnpack (gemma3-1b|xnnpack|--quantize) / linux-job (gh)
  RuntimeError: Command docker exec -t 40b65afd35f48b7a4cdc1cb17e1cd46c7ace04e559f12a49295f8f4551e42a70 /exec failed with exit code 1
• trunk / test-huggingface-transformers-xnnpack (llama3.2-1b|xnnpack|--quantize) / linux-job (gh)
  RuntimeError: Command docker exec -t ffc04c64ae6f9b37d00d07faf8c41702e4065fc98cbb4f985559c5c43d63dfc2 /exec failed with exit code 1
• trunk / test-huggingface-transformers-xnnpack (qwen3-0.6b|xnnpack|--quantize) / linux-job (gh)
  RuntimeError: Command docker exec -t b3264fef2673f1c73f67edf04243712bf4a06006d311f962214b615fe5d97a2e /exec failed with exit code 1
• trunk / test-models-macos-cpu (resnet50, xnnpack-quantization-delegation) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / unittest-release / linux / linux-job (gh)
  examples/models/test/test_export.py::ExportTest::test_efficient_sam_export_to_executorch

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• Test CoreML Backend / test-coreml / test-backend-macos (coreml, models) / macos-job (gh)
  ##[error]The operation was canceled.
• trunk / test-models-macos-cpu (llama2, xnnpack-quantization-delegation) / macos-job (gh)
  ##[error]The operation was canceled.

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

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

• pull / test-samsung-models-linux / linux-job (gh) (trunk failure)
  test_w2l_fp16
• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• trunk / unittest-release / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[SaoirseARM]

@pytorchbot label ciflow/trunk

[SaoirseARM]

@pytorchbot label 'partner: arm'

[SaoirseARM]

@pytorchbot label "release notes: arm"