feat: Add vLLM dtype configuration for GRPO trainer

[joey00072]

What does this PR do?

Adds datatype argument for vllm in grpo Trainer/Config,

T4 in collab need fp16 and vllm defaults to bf16 :)

Fixes # (issue)

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@qgallouedec

[qgallouedec]

Thank, it seems like the doc isn't complete, can you double check?