Support for rerank and colbert

[rjmalagon]

Rerank models are very useful to empower RAG, help a lot with search on RAG and they are resource intensive. It would be very nice to accelerate rerank via llama.cpp, to make it accessible just like embedding.

Colbert models are a more complex tool, between rerank and embedding, but at the end, just an optimized alternative to rerank, very welcome if supported by llama.cpp too.

Actual implementations are strictly transformers based.

https://huggingface.co/mixedbread-ai/mxbai-rerank-large-v1
https://huggingface.co/mixedbread-ai/mxbai-colbert-large-v1

This could allow Open-webui to offload this to Ollama. (open-webui+ollama , maybe the most accessible tools for local RAG)

[ciekawy]

I did open feature request for reranking #8555