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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❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• Long queue for ROCM runners, also B200 and XPU queueing is observed

❌ 1 New Failure, 2 Unrelated Failures

As of commit 161b60b with merge base ed91b6a ():

NEW FAILURE - The following job has failed:

• trunk / test-llama-runner-mac (fp32, coreml) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

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

• trunk / unittest-release / linux / linux-job (gh) (trunk failure)
  [ FAILED ] WavLoaderTest.LoadAudioDataRejectsUnsupportedFormat
• trunk / unittest-release / macos / macos-job (gh) (trunk failure)
  [ FAILED ] WavLoaderTest.LoadAudioDataRejectsUnsupportedFormat

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[SaoirseARM]

@pytorchbot label "release notes: none"

[SaoirseARM]

@pytorchbot label ciflow/trunk

[SaoirseARM]

@pytorchbot label 'partner: arm'