[Misc] Fix the benchmark's README and improve the error messages for the benchmark's argument checks

[tanruixiang]

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Purpose

Fix the benchmark's README, the current multimodal-related example parameters are incorrect.
Improve the error messages for the benchmark's argument checks (Currently, the error message incorrectly lists the --endpoint-type parameter as --backend)

Test Plan

Test Result

(Optional) Documentation Update

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[gemini-code-assist]

Code Review

This pull request correctly fixes incorrect parameters in the benchmark's README examples and improves the clarity of an error message for the benchmark's argument checks. The changes accurately add the --endpoint-type parameter to the examples and update the error message to refer to endpoint-type instead of backend. These changes improve the usability and correctness of the benchmark documentation and error reporting.

[tanruixiang]

@DarkLight1337 PTAL

[DarkLight1337]

I think backend isn't actually used in vllm bench command, can you verify this? If so let's just remove --backend completely

[DarkLight1337]

cc @mgazz @mgoin

[tanruixiang]

@DarkLight1337
--backend was introduced by #20083.
Just for check whether the backend was compatible with the SampleParams.

    if sampling_params and args.backend not in OPENAI_COMPATIBLE_BACKENDS:
        raise ValueError("Sampling parameters are only supported by "
                         "openai-compatible backends.")

It appears to be different from endpoint-type, maybe different backends support the same endpoint-type.
I guess it needs to be retained to support different backends, such as tgi. 🤔

[DarkLight1337]

I am a bit confused by this because I see --endpoint-type inside the old benchmarks/*.py having a very similar purpose as current --backend in vllm bench serve. See if others could provide more insights.

[DarkLight1337]

Anyway let's merge this first to solve the issue

[tanruixiang]

@DarkLight1337 It seems that the CI error is unrelated to this PR?🤔

[DarkLight1337]

Yeah unrelated, let me force merge