### What this PR does / why we need it?
This PR adds vllm bench common method, we need it to add some test cases
later
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
by running the test
- vLLM version: release/v0.13.0
- vLLM main:
5fbfa8d9ef
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Signed-off-by: jiangyunfan1 <jiangyunfan1@h-partners.com>