Fix moondream support

[abetlen]

• Re-applies changes from add support for moondream vision language model #6899
• Fixes inconsistent num_positions
• Correclty zeros embeddings tensor when class embeddings are used

Currently works correctly for original Llava models but moondream and other siglip based models exhibit the same issues as reported in #7060 where subsequent requests with new images are broken. I suspect that the issue is with n_img_pos because what I observe is parts of the previous image patches linger in the context (as observed in model responses).

Closes #7060

[abetlen]

Update: Tested the current PR with a few different models and it actually fixes the bug in almost all cases.

Test Procedure:

1. Load first image and ask model to describe the image with temperature 0.
2. Repeat step 1 and ensure generation is the same.
3. Load second image and ask model to describe the image with temperature 0.
4. Repeat step 3 and ensure generation is the same.

Tested and Working

• LLaVA 1.5
• LLaVA 1.6
• nanoLLaVA
• llama-3-vision-alpha

Tested and Not Working

• Moondream2: Fails right away at step 2, second generation is different from the first.

What's strange is that the nanoLLaVA and llama-3-vision-alpha image projector ggufs follow the same architecture as moondream2 (in fact I generated them by adapting @vikhyat create_gguf.py file) so not sure why this issue only effects that model.

[abetlen]

Update: I'm dumb, so moondream also works correctly with this PR, there was a seperate bug in llama-cpp-python. I was failing to clear the kv cache when the image embedding came before any text.

Tested with llama-cpp-python and llava-cli and it all works on my end.

[ggerganov]

Thanks for taking a look