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>

tests/test-llama-archs.cpp

@@ -357,6 +357,7 @@ static bool moe_mandatory(const llm_arch arch) {
         case LLM_ARCH_KIMI_LINEAR:
         case LLM_ARCH_STEP35:
         case LLM_ARCH_MISTRAL4:
+        case LLM_ARCH_MELLUM:
             return true;
         default:
             return false;