clip: move model cgraphs into their own files

[ngxson]

Some models like deepseek-ocr (#17400), gemma3n (#17961) and LFM2-audio (#17694) adds quite a lot of code into clip.cpp making it hard to track changes.

This PR moves cgraph builder of all vision/audio models into their own file, mirroring the recent refactoring in libllama by @pwilkin

Migration guide for pending PRs

1. Create a new file under mtmd/models/your_model.cpp
2. Implement the code inside newly created file (simply copy your existing build_*() function from clip.cpp into here)
   NOTE: You won't be able to access clip_ctx directly from here. However, you can add custom fields into clip_graph and modify its constructor inside clip.cpp to copy fields into clip_graph
3. Add class definition into models.h
4. Modify clip_image_build_graph() in clip.cpp to use the new cgraph

[ngxson]

Test results:

[vision] OK:   ggml-org/SmolVLM-500M-Instruct-GGUF:Q8_0
[vision] OK:   ggml-org/SmolVLM2-2.2B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/SmolVLM2-500M-Video-Instruct-GGUF:Q8_0
[vision] OK:   ggml-org/gemma-3-4b-it-GGUF:Q4_K_M
[vision] OK:   THUDM/glm-edge-v-5b-gguf:Q4_K_M
[vision] OK:   second-state/Llava-v1.5-7B-GGUF:Q2_K
[vision] OK:   cjpais/llava-1.6-mistral-7b-gguf:Q3_K_M
[vision] OK:   ibm-research/granite-vision-3.2-2b-GGUF:Q4_K_M
[vision] OK:   second-state/MiniCPM-Llama3-V-2_5-GGUF:Q2_K
[vision] OK:   openbmb/MiniCPM-V-2_6-gguf:Q2_K
[vision] OK:   openbmb/MiniCPM-o-2_6-gguf:Q4_0
[vision] OK:   bartowski/Qwen2-VL-2B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/Qwen2.5-VL-3B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/InternVL2_5-1B-GGUF:Q8_0
[vision] OK:   ggml-org/InternVL3-1B-Instruct-GGUF:Q8_0
[vision] OK:   ggml-org/Qwen2.5-Omni-3B-GGUF:Q4_K_M
[vision] OK:   ggml-org/LFM2-VL-450M-GGUF:Q8_0
[vision] OK:   ggml-org/granite-docling-258M-GGUF:Q8_0
[vision] OK:   ggml-org/LightOnOCR-1B-1025-GGUF:Q8_0
[audio]  OK:   ggml-org/ultravox-v0_5-llama-3_2-1b-GGUF:Q8_0
[audio]  OK:   ggml-org/Qwen2.5-Omni-3B-GGUF:Q4_K_M
[audio]  OK:   ggml-org/Voxtral-Mini-3B-2507-GGUF:Q4_K_M
[vision] OK:   ggml-org/pixtral-12b-GGUF:Q4_K_M
[vision] OK:   ggml-org/Mistral-Small-3.1-24B-Instruct-2503-GGUF
[vision] OK:   ggml-org/Qwen2-VL-2B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/Qwen2-VL-7B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/Qwen2.5-VL-3B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/Qwen2.5-VL-7B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/Qwen3-VL-2B-Instruct-GGUF:Q8_0
[vision] OK:   ggml-org/InternVL3-8B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/InternVL3-14B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/Qwen2.5-Omni-7B-GGUF:Q4_K_M
[audio]  OK:   ggml-org/ultravox-v0_5-llama-3_1-8b-GGUF:Q4_K_M
[audio]  OK:   ggml-org/Qwen2.5-Omni-7B-GGUF:Q4_K_M
[vision] OK:   ggml-org/Qwen2.5-VL-72B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/Llama-4-Scout-17B-16E-Instruct-GGUF:IQ1_S

[ggerganov]

The filenaming convention is to use dashes instead of underscores for C++ source files