Arm backend: Skip concating single tensors in AdaptiveAvgPool

[Erik-Lundell]

It was inserting no-op concats.
It was discovered in #18500

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

[pytorch-bot]

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❌ 2 New Failures, 3 Unrelated Failures

As of commit a0b522e with merge base ddd62c5 ():

NEW FAILURES - The following jobs have failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t ab045b3a576031c549b614c0d262022c315358f65b9afdb3273fbd76710af664 /exec failed with exit code 139
• trunk / test-qnn-model (fp32, mb) / linux-job (gh)
  AttributeError: 'DataFrame' object has no attribute 'Conference'

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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