[Benchmark] Add hf_stream arg to enable or disable datasets streaming loading

[Potabk]

This pr adds hf_stream arg for benchmark_serving to enable or disable datasets streaming loading, Because some users have to load the dataset from local due to network reasons, we added this parameter to let users decide whether to enable streaming loading

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[Potabk]

@JenZhao would mind have a review

[cjackal]

I don't think this would work as intended; as we set action="store_true", args.hf_stream is always True regardless of user passing --hf-stream option or not.

[Potabk]

thx for remind, should not set a default value