[Core] *Prompt* logprobs support in Multi-step

[afeldman-nm]

Fix the bug that causes vLLM to crash when configuring non-None prompt logprobs.

Test:

pytest -s -v tests/multi_step/test_correctness_llm.py::test_multi_step_llm_w_prompt_logprobs[5-5-10-8-True-5-1-half-JackFram/llama-160m]

FIX #8198

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[afeldman-nm]

CC: @comaniac @SolitaryThinker @robertgshaw2-neuralmagic @alexm-neuralmagic @njhill

Actually ready for review this time :)

[comaniac]

Overall LGTM. Only nits.
Also cc @SolitaryThinker @njhill