Arm backend: Update epsilon to correct default of 2**-12

[SaoirseARM]

Summary

Epsilon was set too high in the test pipeline. Fix this to be the correct default of: 2**-12

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/16096

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✅ You can merge normally! (3 Unrelated Failures)

As of commit 850eb29 with merge base 4014597 ():

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

• trunk / test-huggingface-transformers-macos (olmo-1b|coreml_fp32_gpu|--quantize) / macos-job (gh) (detected as infra flaky with no log or failing log classifier)
• trunk / test-models-linux-aarch64 (add, xnnpack-quantization-delegation, linux.arm64.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128

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

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

• pull / android / run-emulator (gh) (trunk failure)
  Timeout waiting for emulator to boot.

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[SaoirseARM]

@pytorchbot label 'partner: arm'

[SaoirseARM]

@pytorchbot label ciflow/trunk

[SaoirseARM]

@pytorchbot label "release notes: none"