Arm backend: Xfail RIFE VGF tests with lazy import

[JakeStevens]

Summary

Reapplies #21755 after the revert in #21783, but imports the model-converter version helpers lazily during test collection. This avoids importing torch from the Arm conftest before internal test infrastructure has initialized its symbols, while preserving the RIFE VGF quant test xfail behavior.

The import is attempted only when matching RIFE VGF quant tests are collected.

Test plan

CI

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell @rascani

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21786

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❌ 2 New Failures, 1 Unclassified Failure

As of commit 0fe10ac with merge base c6213ae ():

NEW FAILURES - The following jobs have failed:

• Apple / build-demo-ios / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 65
• pull / test-arduino-library / run / arduino-library (gh)
  RuntimeError: Command docker exec -t 468e4caf31916f8116ce128bd089b0d908d3303fb47f05cea78bc77c63dd754f /exec failed with exit code 1

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• trunk / test-models-macos-coreml (ic3) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

This comment was automatically generated by Dr. CI and updates every 15 minutes.

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[meta-codesync]

@JakeStevens has imported this pull request. If you are a Meta employee, you can view this in D115750356.

[digantdesai]

Review automatically exported from Phabricator review in Meta.