Add backward compatibility for GuidedDecodingParams

[hmellor]

Reinstates backward compatiblility for the following APIs removed by #22772:

• Adds GuidedDecodingParams which wraps StructuredOutputsParams (this was just a rename)
• Add guided_decoding back to SamplingParams/SamplingParams.from_optional and forward the value to structured_outputs_params

[gemini-code-assist]

Code Review

This pull request adds backward compatibility for GuidedDecodingParams by reintroducing it as a deprecated alias for StructuredOutputsParams. The changes are logically sound for ensuring compatibility. However, the implementation introduces duplicated code for issuing deprecation warnings, which can impact maintainability and user experience. My review focuses on removing this redundancy.

[aarnphm]

Fo we need this for 0.11?

im also good with breaking change

[hmellor]

We've been made aware that having the breaking change this sudden will causee a lot of breakages for RL use cases.

As backwards compatibility goes, this is pretty unobtrusive to vLLM and very loud with warnings to encourage users to switch.

[russellb]

This seems important and aligned with our deprecation policy: https://docs.vllm.ai/en/latest/contributing/deprecation_policy.html#deprecation-pipeline

It's actually proposing to skip the "Deprected + off by default" step, but I'm OK with that. I think that part is more important for HTTP API breakages, which are harder to inform the client about.

[facebook-github-bot]

@minosfuture has imported this pull request. If you are a Meta employee, you can view this in D83067559.