fix: Update models docs' content and repair the import

[neverbiasu]

This pull request addresses the following issues:

• Content Update: Improved the description of CLIP to accurately reflect its role as a multimodal vision and language model. Enhanced the explanation of diffusion models for better clarity and adjusted file size units to use "GB" for better internationalization.
• Import Path Repair: Fixed the incorrect import path for ExternalModels to ensure proper functionality and avoid errors.

[comfyui-wiki]

Thanks for the PR