fix(model): Fix vllm new tokenizer error

[fangyinc]

Description

Closes #1600

How Has This Been Tested?

Modify your .env file:

LLM_MODEL=qwen-1.8b-chat
MODEL_TYPE=vllm

Restart DB-GPT.

Snapshots:

Include snapshots for easier review.

Checklist:

• My code follows the style guidelines of this project
• I have already rebased the commits and make the commit message conform to the project standard.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• Any dependent changes have been merged and published in downstream modules

[Aries-ckt]

r+

[csunny]

r+