Arm backend: Add int16x8 LayerNorm test cases

[SaoirseARM]

Summary

• Updates test_rsqrt to use a lower epsilon
• Adds epsilon parameter to test_pipeline.py

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/16015

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❌ 1 New Failure, 269 Pending

As of commit 01329f7 with merge base a47a122 ():

NEW FAILURE - The following job has failed:

• Apple / build-demo-ios / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1

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[SaoirseARM]

@pytorchbot label ciflow/trunk

[SaoirseARM]

@pytorchbot label 'partner: arm'

[github-actions]

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