[Frontend] Add --logprobs argument to benchmark_serving.py

[afeldman-nm]

(1) benchmarks/benchmark_serving.py now supports a --logprobs CLI argument, and (2) the value of the --logprobs CLI argument configures the underlying logprobs argument to the completion requests generated during benchmarking.

python benchmarks/benchmark_serving.py     --backend vllm      --model JackFram/llama-160m     --dataset-name sharegpt --logprobs 4  --dataset-path ../sharegpt.json

FIX #8193 (link existing issues this PR will resolve)

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[github-actions]

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🚀

[afeldman-nm]

/ready

[afeldman-nm]

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

[comaniac]

LGTM. Just nits.

[afeldman-nm]

/ready