Update backends-xnnpack.md

[metascroy]

Update XNNPACK docs

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❌ 2 New Failures

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NEW FAILURES - The following jobs have failed:

• Check Labels / Check labels (gh)
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• pull / unittest-arm / linux-job (gh)
  RuntimeError: Command docker exec -t 8c3c62105ce1f27f7c595e5d8cd0c78044bce74495699c1f9b23051987473004 /exec failed with exit code 1

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