model: add Mellum architecture (#23966)
* model: support for Mellum architecture
* model: improve mellum.py formatting
* model: improve mellum.py formatting once again
* deps: downgrade transformers to 4.57.6 (to fix CI)
* deps: remove huggingface_hub dependency
* deps: remove huggingface_hub from test requirements
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Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>
