model: support arch `DbrxForCausalLM` (#6515)

* model: dbrx convert to gguf
#6344

* llama: support dbrx
#6344

* doc: dbrx: add the model as supported

* scripts: get-wikitext-2 add unzip

* llama: increase maximum experts allowed

* llama: factorize moe graph implementation between grok, mixtral and dbrx


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Co-authored-by: Megha Agarwal <16129366+megha95@users.noreply.github.com>