### What problem does this PR solve?

The current design is not well-suited for multimodal models, as each
model can only be configured for a single purpose—either chat or
Img2txt. To work around this limitation, we use model aliases such as
gpt-4o-mini and gpt-4o-mini-2024-07-18.

To fix this, this PR allows specifying the Img2txt model by tag instead
of model_type.

### Type of change
- [x] Refactoring