Add prefill method support without generation

[craftingmod]

Summary

• Add create_chat_prefill method which could be used to prompt evaluation.
  • Expose a prefill-only chat API that evaluates and returns the final next-token without generation.
• Add chat_template_kwargs support in create_chat_completion for overriding chat_template_kwargs each completion.
  • can override extra_template_arguments value of each completion.

Maybe useful when implementing probability classification.

AI Disclose

I've used GPT 6 Luna to implementing features, but I manually reviewed and understood how to works.

[craftingmod]

• Why i made this PR: I want trying something like system-one in comfyui 😕
• Code quality might be bad so feel free to edit or close PR 😉