[Bug]: Inaccurate x2v benchmark results

Your current environment

Your output of `python collect_env.py` here

Your code version

🐛 Describe the bug

The diffusion_benchmark_serving.py script does not enable the guidance scale parameter. The default guidance scale parameter causes the benchmark to only run positive tests during the low-noise phase, omitting negative tests, resulting in inaccurate latency measurements.

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