Synthetic rollout length for GRPO performance benchmarking

**Is your feature request related to a problem? Please describe.**
Vllm benchmarking CLI supports ignoring EOS and synthesizing given OSL ([--random-output-len ](https://docs.vllm.ai/en/latest/cli/bench/serve.html?h=ignore_eos#-random-output-len)). Can we use this feature to generate synthetic benchmarks of fixed OSL for evaluating perf at certain sequence length scenario?

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