Update vLLM version support to 0.19.0

[sergiopaniego]

What does this PR do?

 pytest tests/test_vllm_client_server.py
=========================================================================================================== test session starts ===========================================================================================================
platform linux -- Python 3.11.13, pytest-9.0.3, pluggy-1.6.0
rootdir: /fsx/sergiopaniego/trl
configfile: pyproject.toml
plugins: rerunfailures-15.1, datadir-1.8.0, anyio-4.12.1, xdist-3.8.0, cov-7.1.0
collected 51 items                                                                                                                                                                                                                        

tests/test_vllm_client_server.py ..................x..................sssssssss.....                                                                                                                                                [100%]

===================================================================================== 41 passed, 9 skipped, 1 xfailed, 1 warning in 531.07s (0:08:51) =====================================================================================

https://github.com/vllm-project/vllm/releases/tag/v0.19.0

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? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

AI writing disclosure

We welcome the use of AI tools to help with contributions. For transparency and to help us improve our review process, please indicate the level of AI involvement in this PR.

• No AI usage: the PR was written entirely by a human.
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Who can review?

@qgallouedec @albertvillanova

---

Note

Low Risk
Version ceiling and messaging only; no trainer or vLLM integration logic changed.

Overview
Raises the supported vLLM upper bound from 0.18.0 to 0.19.0 everywhere TRL documents and enforces that range.

The optional vllm extra in pyproject.toml now pins vllm>=0.12.0,<=0.19.0. is_vllm_available() in trl/import_utils.py uses the same range for its compatibility warning. The vLLM integration doc warning block matches the new ceiling.

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[HuggingFaceDocBuilderDev]

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[albertvillanova]

Thanks!