Arm backend: Addressing Linear operator issues in int16x8

[SaoirseARM]

Summary

• Update bias to use the derived quantization spec to int32.
• Fixes issue in the Conv2d where the bias should be added first and then rescaled back to int16.

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15370

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❌ 9 New Failures, 3 Unrelated Failures

As of commit aba691a with merge base 69f79b9 ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 7d038a09dac650c0c02a1767827e9f60e6b2bdf30cf58af339e3f5968eb42f0c /exec failed with exit code 1
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 6921ed5d87f22689d7ba684491db182ad33149aa837b9ec2c8e2e817ebb0c62c /exec failed with exit code 1
• Test CUDA Builds / export-gemma3-cuda-non-quantized / linux-job (gh)
  RuntimeError: Command docker exec -t 38afc261122c286af80bd3cf709313632bb3cc7e70fcd494d940af265b757a20 /exec failed with exit code 2
• Test CUDA Builds / export-gemma3-cuda-quantized-int4-tile-packed / linux-job (gh)
  RuntimeError: Command docker exec -t 6d024cd67fbaf6c7d20c7f0977168dbe25821f73ea69231b1260cb67be016621 /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-non-quantized / linux-job (gh)
  RuntimeError: Command docker exec -t e006e5b7b526a815c3d9dc0abde6dd453d2cca982f428f60e0ed89527622a70b /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-quantized-int4-tile-packed / linux-job (gh)
  RuntimeError: Command docker exec -t a5242164e3877539928809e0c4ea29a30040833031666835d2fc4f0e4f89dc81 /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-quantized-int4-weight-only / linux-job (gh)
  RuntimeError: Command docker exec -t befff33b6f08abb98607e84fb80825df745e21ba0abd8f0ff750d90a0062f55f /exec failed with exit code 2
• Test Metal Backend / export-voxtral-metal-artifact / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 2
• trunk / test-qnn-model (fp32, vit) / linux-job (gh)
  RuntimeError: Command docker exec -t 3a41d00d104c5b1fef99041a8d03b63683a61fcea2c9f657e46504ae41ead4c6 /exec failed with exit code 1

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / unittest / linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• trunk / unittest-release / linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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

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

• pull / unittest-editable / linux / linux-job (gh) (trunk failure)

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[SaoirseARM]

@pytorchbot label "release notes: none"

[SaoirseARM]

@pytorchbot label ciflow/trunk

[SaoirseARM]

@pytorchbot label 'partner: arm'