mtmd : add post-decode callback (#24645)

Assisted-by: pi:llama.cpp/Qwen3.6-27B

tools/mtmd/mtmd-helper.h

@@ -91,6 +91,8 @@ MTMD_API int32_t mtmd_helper_eval_chunk_single(mtmd_context * ctx,
                                                bool logits_last,
                                                llama_pos * new_n_past);
 
+typedef int32_t (*mtmd_helper_post_decode_callback)(struct llama_batch batch, void * user_data);
+
 // helper function to decode an image whose embeddings have already been calculated
 // this helper will handle batching and pre/post decoding setup (for ex. gemma 3 requires non-causal attention)
 // ret 0 on success, -1 on chunk not being a valid image chunk, 1 on decode failure
@@ -101,7 +103,9 @@ MTMD_API int32_t mtmd_helper_decode_image_chunk(mtmd_context * ctx,
                                                 llama_pos n_past,
                                                 llama_seq_id seq_id,
                                                 int32_t n_batch,
-                                                llama_pos * new_n_past);
+                                                llama_pos * new_n_past,
+                                                mtmd_helper_post_decode_callback callback,
+                                                void * user_data);
 
 //
 // video input helpers (requires ffmpeg/ffprobe installed on the system)