[rollout, vllm, sglang] fix: set default max_model_len

[ji-huazhong]

What does this PR do?

Only assign max_model_len from hf_config if it's currently None to avoid overwriting user settings.

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[gemini-code-assist]

Code Review

This pull request fixes an issue where a user-provided max_model_len was always being overwritten by the value from the model's configuration. The change correctly ensures that the user's setting is respected. However, this also removes an implicit validation. I've added a suggestion to warn the user if they provide a max_model_len that exceeds the model's supported length, as this could lead to errors or unexpected behavior.

[vermouth1992]

Please modify sglang as well

[ji-huazhong]

done