tests: unskip and fix offline mode test using HF_HUB_OFFLINE + hermetic cache warmup

[arvinder004]

What does this PR do?

The offline test was skipped and brittle due to reliance on TRANSFORMERS_OFFLINE and pre-warmed caches. Standardize on HF_HUB_OFFLINE and make the test self-contained and deterministic.

Fixes #41311

Changes:

• Unskip test_offline_mode.
• Implement two-step subprocess approach:
  • Step 1: Warm cache online by loading config, model, tokenizer, and a small pipeline.
  • Step 2: Run offline with HF_HUB_OFFLINE=1 and a socket.socket monkey-patch to fail on any network access; load everything from local cache.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?
  • Updated tests/utils/test_offline.py::OfflineTests::test_offline_mode to be hermetic (two-step subprocess: cache warmup online, then offline with HF_HUB_OFFLINE=1 and socket mock).

How to validate

Run: pytest -q tests/utils/test_offline.py::OfflineTests::test_offline_mode

@Cyrilvallez
@Rocketknight1
@ydshieh

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: gpt2