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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❗ 1 Active SEVs

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• Rolling out OSDC (ARC) runners on pull workflow for PyTorch trunk commits

❌ 4 New Failures, 1 Cancelled Job, 3 Unrelated Failures

As of commit 7d4ffa1 with merge base 2eaa16c ():

NEW FAILURES - The following jobs have failed:

• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model
• trunk / test-torchao-huggingface-checkpoints (lfm2_5_1_2b, linux.arm64.2xlarge, executorch-ubuntu-22.04-g... / linux-job (gh)
  RuntimeError: Command docker exec -t ecd1b4c85d68db0740525a06cafe4eff3f42552cd1b94a73cc70c0c2ac2b61a6 /exec failed with exit code 1
• trunk / test-torchao-huggingface-checkpoints (phi_4_mini, linux.arm64.2xlarge, executorch-ubuntu-22.04-gc... / linux-job (gh)
  RuntimeError: Command docker exec -t 087197a0a81066b4121ff817f4f31e186cae85c8ed19856ffcc1e7aca274fd8c /exec failed with exit code 1
• trunk / test-torchao-huggingface-checkpoints (qwen3_4b, linux.arm64.2xlarge, executorch-ubuntu-22.04-gcc1... / linux-job (gh)
  RuntimeError: Command docker exec -t 0458c2d144fb0905e078a59949f5e6eb2ec73b35d2a0952883f135406b8d8be6 /exec failed with exit code 1

CANCELLED JOB - The following job was cancelled. Please retry:

• trunk / test-selective-build-macos (cmake) / 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 / 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"

[SaoirseARM]

Failures look unrelated.