Add vlm document

[zhaochenyang20]

Motivation

I added the VLM API documents. Please check it carefully, especially the chat template part.

I swear I will learn to use git rebase tomorrow to keep my future PRs neat and tidy. o(╥﹏╥)o

Modifications

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.