Arm backend: Fix flaky sigmoid 16/32 bit tests

[Erik-Lundell]

• Change constant test data to random generators.
• Tests on Ethos-U55 are meant to xfail as int 16 tables are currently not supported.
• For other tests, add flaky marker. Remove increased qtol, since the inaccuracies only show up sporadically.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

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❌ 3 New Failures

As of commit 00d5d79 with merge base 8cd1b93 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner / linux-job (gh)
  >>> Lint for examples/models/moshi/mimi/test_mimi.py:
• pull / test-llava-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 113fe954cfd0d1cf3710306b747396c2a7568736bb17abfcfb77bd64cbdb993a /exec failed with exit code 139
• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 11

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[zingo]

Fails are unrelated