check user-specified model_max_len with hf derived max_model_len

[BBuf]

Motivation

This PR aims to improve the handling of context length in the ModelConfig class. It addresses potential issues when users specify a context length exceeding the model's derived maximum, enhances error messaging, and introduces a controlled override mechanism. These changes will prevent decoding nan errors, which I have met recently.

Modifications

The ModelConfig class in model_config.py has been updated to include a check comparing user-specified context_length against the model's derived_context_len. It now handles the SGLANG_ALLOW_LONG_MAX_MODEL_LEN environment variable, allowing controlled overrides of maximum context length. The changes include logic to raise a ValueError or print a warning based on the specified length and override settings, with improved error and warning messages for clear user guidance.

Checklist

• Format your code according to the Contributor Guide.
• Add unit tests as outlined in the Contributor Guide.
• Update documentation as needed, including docstrings or example tutorials.

[merrymercy]

Thanks for the contribution. Please fix the lint errors and my comments.

[merrymercy]

please fix the lint error https://github.com/sgl-project/sglang/blob/main/docs/en/contributor_guide.md#format-your-code

[BBuf]

please fix the lint error https://github.com/sgl-project/sglang/blob/main/docs/en/contributor_guide.md#format-your-code

done!