Add TTFT to offline_inference_with_prefix.py

[xu-song]

TTFT is an important metric for prefix cache

[github-actions]

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[comaniac]

TTFT is not usually used in offline inference which cares more about throughput than latency.

[xu-song]

As I know, the throughput benefit mainly comes from TTFT in prefilling period, not token/s in decoding period. Is that right?

[comaniac]

Yes but that's not my point. TTFT is only meaningful in online serving with streaming output. In offline inference, we don't output responses token by token, so we usually rely on end to end throughput or "input (prefill)" throughput as metrics. If you want know see the TTFT improvements with prefix caching, we also have a serving benchmark for it.