[Bugfix] Add seed to benchmark_serving.py

[ekagra-ranjan]

Following #17929, this PR uses seed for benchmark_serving script. Will be used for TP > 1 bench of SD method like this #17202

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[DarkLight1337]

Is it intended that you set the seed per request instead of at startup time? The previous PR only adjusts the behavior for setting it at startup time.

[ekagra-ranjan]

you are right. Closing this PR