convert : experimental support for `--mmproj` flag (#13023)
* convert : experimental support for `--mmproj` flag
* fix bad ctrl+f replace
* fix style
* split into subclasses TextModel and VisionModel
* rename Mode --> ModelBase
* small fix
* correct CLIP_VISION arch name (because existing GGUF already use it)
* Apply suggestions from code review
Co-authored-by: compilade <git@compilade.net>
* fix Mistral3Model
* fix typo
Co-authored-by: compilade <git@compilade.net>
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Co-authored-by: compilade <git@compilade.net>
