Arm backend: fix Torch compatibility

[swha815]

Summary

• Recent pin update to Torch 2.12 no longer exposes .out overloads for quantized-decomposed operator.
• Code for backward-compatiliby is included, but should be removed in the future when older Torch versions are no longer supported.

Test plan

Tested using Arm-internal CI.

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

[pytorch-bot]

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❌ 36 Pending, 2 Unclassified Failures

As of commit 8efd474 with merge base 1b26db1 ():

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:

• trunk / test-huggingface-transformers-macos (llama3.2-1b|coreml_fp32_gpu|--quantize) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  The process '/opt/homebrew/bin/git' failed with exit code 1
• trunk / test-llama-runner-mac (fp32, xnnpack+custom+quantize_kv) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  ##[error]The operation was canceled.

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