[Bugfix] Set SamplingParams.max_tokens for OpenAI requests if not provided by user

[zifeitong]

FIX #6707

vllm/engine/output_processor/multi_step.py requires SamplingParams.max_tokens to be set.

AFAICS, all the requests without max_tokens (the default) will fail if spec_decoding is turned on now.

The bug is introduced in #4028 in which _validate_prompt_and_tokenize() is moved behind request.to_sampling_params(). _validate_prompt_and_tokenize() which calls _validate_input() will rewrite the request and set max_tokens.

In this PR, the max_tokens is set explicitly outside _tokenize_prompt_input.

I will create another PR to allow multi step output processor accept requests without max_tokens.

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[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
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[zifeitong]

/ready

[DarkLight1337]

My bad, thanks for fixing this! Just a small comment.