llava-cli: improve llava-cli and the API for using LLaVA

[jakexcosme]

Note: This issue was copied from ggml-org#6027

Original Author: @phymbert
Original Issue Number: ggml-org#6027
Created: 2024-03-12T21:18:55Z

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From:

• server : improvements and maintenance ggml-org/llama.cpp#4216 (comment)

1. cleaning up the clip/llava libs and improving the API
2. in the old implementation, there were many internal object exposed to the server and the memory management was dubious
3. there was no obvious path for supporting parallel multimodal slots