mtmd: add batching API

[ngxson]

Overview

Supersede #24300

Also fix #24380

Add a generic batching API to mtmd and wire it up to llama-server, the goal is to speed up llava-uhd-style models and at the same time, improve video processing speed

Current state:

• llama-server can use it correctly
• mtmd API implement is mock up, need to implement the proper logic

TODO:

• add notion of max batch size in mtmd
• add CLI argument for it
• mtmd_batch_add_chunk should only accept input with same size
• wire up mtmd_batch_encode to use the 4th batch dim, added via mtmd: build_vit batching #24352
• blacklist / whitelist models that can support it --> maybe only support build_vit() models for now
• maybe update mtmd-cli to reflect the usage --> not sure, maybe a follow-up PR is better

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: no