clip : fix confused naming ffn_up and ffn_down

[ngxson]

The old clip.cpp code reverse the ffn_up and ffn_down naming by mistake, which make it extremely messy when migrating the conversion script to convert_hf_to_gguf.py

To save myself from some headaches, so I decided to fix it once and for all 😤😤

The new rule is:

• New model always have fc1 == ffn_up and fc2 == ffn_down
• For old models, if the reversed ffn naming is detected, we reverse it back

This PR also contain name changing to make it more align with llama.cpp style:

• n_intermediate --> n_embd
• hidden_size --> n_ff

Small note: GGUF converted from the old qwen surgery script has n_ff = 0, hopefully this will not be messy in the future

---

Tested by converting fresh new GGUF and run it with llama-mtmd-cli locally:

• Gemma 3 4B
• SmolVLM2 2.2B
• Qwen 2.5 3B

Tested with existing GGUF on the internet:

OK:   llama-mtmd-cli ggml-org/SmolVLM-500M-Instruct-GGUF:Q8_0
OK:   llama-mtmd-cli ggml-org/SmolVLM2-2.2B-Instruct-GGUF:Q4_K_M
OK:   llama-mtmd-cli ggml-org/SmolVLM2-500M-Video-Instruct-GGUF:Q8_0
OK:   llama-mtmd-cli ggml-org/gemma-3-4b-it-GGUF:Q4_K_M
OK:   llama-mtmd-cli guinmoon/MobileVLM-3B-GGUF:Q4_K_M
OK:   llama-mtmd-cli THUDM/glm-edge-v-5b-gguf:Q4_K_M
OK:   llama-mtmd-cli second-state/Llava-v1.5-7B-GGUF:Q2_K
OK:   llama-mtmd-cli cjpais/llava-1.6-mistral-7b-gguf:Q3_K
OK:   llama-mtmd-cli ibm-research/granite-vision-3.2-2b-GGUF:Q4_K_M
OK:   llama-mtmd-cli second-state/MiniCPM-Llama3-V-2_5-GGUF:Q2_K
OK:   llama-mtmd-cli openbmb/MiniCPM-V-2_6-gguf:Q2_K
OK:   llama-mtmd-cli openbmb/MiniCPM-o-2_6-gguf:Q4_0
OK:   llama-mtmd-cli bartowski/Qwen2-VL-2B-Instruct-GGUF:Q4_K_M
OK:   llama-mtmd-cli ggml-org/Qwen2.5-VL-3B-Instruct-GGUF:Q4_K_M
OK:   llama-mtmd-cli ggml-org/pixtral-12b-GGUF:Q4_K_M
OK:   llama-mtmd-cli ggml-org/Mistral-Small-3.1-24B-Instruct-2503-GGUF
OK:   llama-mtmd-cli ggml-org/Qwen2-VL-2B-Instruct-GGUF:Q4_K_M
OK:   llama-mtmd-cli ggml-org/Qwen2-VL-7B-Instruct-GGUF:Q4_K_M
OK:   llama-mtmd-cli ggml-org/Qwen2.5-VL-3B-Instruct-GGUF:Q4_K_M
OK:   llama-mtmd-cli ggml-org/Qwen2.5-VL-7B-Instruct-GGUF:Q4_K_M

[ngxson]

Hi @slaren @ggerganov , sorry for pinging but I realized that some models current does not work correctly without this fix.

If you have time, could you please review this PR? Thanks!