Arm backend: Xfail RIFE VGF tests with lazy import (#21786)

[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

Reviewed By: digantdesai

Differential Revision: D115750356

Pulled By: JakeStevens

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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❌ 3 New Failures, 1 Unrelated Failure

As of commit f724276 with merge base e265ea6 ():

NEW FAILURES - The following jobs have failed:

• Cadence Build & Test / hifi-build / hifi4 (gh)
  ##[error]Refusing to check out fork pull request code from a 'pull_request_target' workflow. This workflow runs with the base repository's GITHUB_TOKEN, secrets, default-branch cache scope, and runner access. Fetching and executing a fork's code in that trusted context commonly leads to "pwn request" vulnerabilities. To opt in, review the risks at https://gh.io/securely-using-pull_request_target and set 'allow-unsafe-pr-checkout: true' on the actions/checkout step.
• Cadence Build & Test / vision-build / vision (gh)
  ##[error]Refusing to check out fork pull request code from a 'pull_request_target' workflow. This workflow runs with the base repository's GITHUB_TOKEN, secrets, default-branch cache scope, and runner access. Fetching and executing a fork's code in that trusted context commonly leads to "pwn request" vulnerabilities. To opt in, review the risks at https://gh.io/securely-using-pull_request_target and set 'allow-unsafe-pr-checkout: true' on the actions/checkout step.
• pull / test-arduino-library / run / arduino-library (gh)
  RuntimeError: Command docker exec -t 6ff0ca6c7a1ed00ca2676f60b489fc0332f664c54cb8452138b073d19b97e60d /exec failed with exit code 1

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

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

• Apple / build-demo-ios / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 65

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[digantdesai]

Review automatically exported from Phabricator review in Meta.

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