[SGLang-Diffusion] Add offline throughput benchmark script for multi-modal models (sgl-project#18154)

Co-authored-by: Hao Jin <Hao Jin>
Co-authored-by: zhaochenyang20 <zhaochen20@outlook.com>
Co-authored-by: Xiaoyu Zhang <35585791+BBuf@users.noreply.github.com>

Author: 3 people