Fix module aliases breaking unittest via __warningregistry__ probes

[tanvir-ux]

What does this PR do?

Fixes #48966.

I hit this while running unrelated unit tests after importing transformers: unittest.TestCase.assertWarns / assertWarnsRegex started raising ModuleNotFoundError: No module named 'torchvision' even when the test never touched image processors.

assertWarns walks every module in sys.modules and probes __warningregistry__. Our legacy *_fast image-processor alias modules implement __getattr__ by forwarding into importlib.import_module(...). That probe therefore imported torchvision-backed processors (e.g. Aria) and failed in environments without torchvision.

I taught both alias __getattr__ paths to raise AttributeError for dunder names without importing the target — the same idea as the existing explicit module.__file__ = None guard for inspect. I also added a regression test that exercises assertWarnsRegex after import and checks the Aria fast alias returns None for __warningregistry__.

Before / after

import unittest
import transformers  # torchvision not installed

class TestWarning(unittest.TestCase):
    def test_warns_regex(self):
        with self.assertWarnsRegex(DeprecationWarning, r"hello"):
            pass

# before: ModuleNotFoundError: No module named 'torchvision'
# after:  AssertionError: DeprecationWarning not triggered

Testing

• Reproduced the failure on current main, applied the patch, and confirmed the snippet above now yields the expected AssertionError.
• Ran tests/utils/test_import_utils.py::test_module_aliases_ignore_dunder_attribute_probes locally — passed.

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