This commit introduces a new configuration option offload_mmproj to the llamacpp extension.
The offload_mmproj setting allows users to control whether the multimodal projector model is offloaded to the GPU. By default, it's offloaded for better performance. If set to false, the projector model will remain on the CPU, which can be useful in low GPU memory scenarios, though image processing might take longer.
Additionally, this commit adds validate_mmproj_path to ensure the provided --mmproj path is valid and accessible, preventing issues during model loading.
This change also refactors some invoke calls for improved readability.
