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

---------

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>

tools/server/tests/requirements.txt

@@ -1,6 +1,5 @@
 aiohttp~=3.9.3
 pytest~=8.3.3
-huggingface_hub>=1.5.0,<2.0
 numpy~=1.26.4
 openai~=2.14.0
 prometheus-client~=0.20.0