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>

requirements/requirements-convert_legacy_llama.txt

@@ -1,7 +1,7 @@
 numpy~=1.26.4
 sentencepiece>=0.1.98,<0.3.0
 
-transformers==5.5.1
+transformers==4.57.6
 
 gguf>=0.1.0
 protobuf>=4.21.0,<5.0.0

requirements/requirements-tool_bench.txt

@@ -1,6 +1,5 @@
 aiohttp~=3.9.3
 pytest~=8.3.3
-huggingface_hub>=1.5.0,<2.0
 matplotlib~=3.10.0
 numpy~=1.26.4
 openai~=2.14.0