Update transformers minimum version to 4.56.1

[albertvillanova]

Update transformers minimum version to 4.56.1.

That patch version includes the fix:

• Fix backward compatibility with accelerate in Trainer transformers#40668

This should fix the CI tests with minimum versions: https://github.com/huggingface/trl/actions/runs/17571124955/job/49907398546

AttributeError: 'Accelerator' object has no attribute 'parallelism_config'

[HuggingFaceDocBuilderDev]

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[albertvillanova]

Most CI tests with minimum versions are green now; only some red left: https://github.com/huggingface/trl/actions/runs/17583974907/job/49946949816

[qgallouedec]

thanks!