Hotfix CI with dev dependencies: xfail test_training_vlm_and_liger

[albertvillanova]

Hotfix CI with dev dependencies: xfail tests/test_grpo_trainer.py::TestGRPOTrainer::test_training_vlm_and_liger[trl-internal-testing/tiny-Qwen2_5_VLForConditionalGeneration]

AttributeError: 'Qwen2_5_VLForConditionalGeneration' object has no attribute 'language_model'

This PR marks test as expected to fail until the upstream fix in liger-kernel is released:

• Fix missing property access for multimodal models linkedin/Liger-Kernel#966
• AttributeError for 'language_model' in transformers v5 linkedin/Liger-Kernel#960

Hotfix for:

• CI fails with dev dependencies: AttributeError: 'Qwen2_5_VLForConditionalGeneration' object has no attribute 'language_model' #4601

The issue appeared in CI tests with dev dependencies after the merge of this PR in transformers:

• 🚨 Generalize get_decoder() for multimodal and delete redundant code 🔪  transformers#42156

Testing improvements:

• Added an xfail marker to the test parameterization for model versions, causing the test to be expected to fail when running with a development release of the transformers library, due to a known issue in the upstream liger-kernel dependency. The marker includes a clear reason and references the relevant upstream issues and required version for resolution.

[HuggingFaceDocBuilderDev]

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[albertvillanova]

CI is completely green! 😅

[qgallouedec]

CI is completely green! 😅

Nicely done!! For how long? 😅