CI hotfix: xfail test_training_with_transformers_paged

[albertvillanova]

CI hotfix: xfail test_training_with_transformers_paged for:

• OnlineDPOTrainer
• GRPOTrainerSlowTester

This PR marks tests as expected to fail for transformers < 4.56.2.

Changes:

• Add a version check for transformers in the test_training_with_transformers_paged test:
  • Mark the test as expected to fail (xfail) for versions below 4.56.2 due to an upstream bug (commented by @kashif in: Fix ValueError in pad function when tensors list is empty #4043 (comment)):
    • [Continous Batching] fix do_Sample=True in continuous batching transformers#40692
  • Fail the test if run on newer versions to remember reverting this PR and remove the xfail mark

Fix #4035.
Supersede and close #4043.

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[kashif]

we can also comment out the GRPO paged attention test as well... it also is "failing"

[albertvillanova]

@kashif which test do you refer?

[kashif]

@albertvillanova this one: https://github.com/huggingface/trl/blob/main/tests/slow/test_grpo_slow.py#L172