Reduce prompt logprobs memory usage

[andrewzhao-mistral]

Summary:

• Reuse the existing compute_topk_logprobs path when computing prompt logprobs in GPUModelRunner.
• Avoid materializing the full log_softmax tensor over the vocabulary for prompt logprobs.
• Preserve the existing target-token-first output ordering expected by LogprobsTensors.

Testing:

• git diff --check -- vllm/v1/worker/gpu_model_runner.py
• python3 -m py_compile vllm/v1/worker/gpu_model_runner.py

[github-actions]

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[njhill]

@andrewzhao-mistral could you switch to use model runner v2? (VLLM_USE_V2_MODEL_RUNNER=1)

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @andrewzhao-mistral.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork