[Feature] Support `return_hidden_states` in async engine

Checklist

• 1. If the issue you raised is not a feature but a question, please raise a discussion at https://github.com/sgl-project/sglang/discussions/new/choose Otherwise, it will be closed.
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Motivation

Currently, it is possible to get hidden states using the offline interface, i.e. Engine.generate, but Engine.async_generate doesn't support this function.

result = await model.async_generate(
        ["hello", "how are you doing?"],
        sampling_params={"temperature": 0, "max_new_tokens": 1},
        return_hidden_states=True)

TypeError: Engine.async_generate() got an unexpected keyword argument 'return_hidden_states'

This can be useful when developing web services where some additional hidden states postprocess logics is done.

Related resources

No response