mtmd: pad mask for qwen2.5vl (#16954)

* mtmd: pad mask for qwen2.5vl

* improve

tools/mtmd/clip.cpp

@@ -761,6 +761,15 @@ struct clip_graph {
             ggml_set_name(window_mask, "window_mask");
             ggml_set_input(window_mask);
 
+            // if flash attn is used, we need to pad the mask and cast to f16
+            if (ctx->flash_attn_type == CLIP_FLASH_ATTN_TYPE_ENABLED) {
+                int n_pad = GGML_PAD(window_mask->ne[1], GGML_KQ_MASK_PAD) - window_mask->ne[1];
+                if (n_pad > 0) {
+                    window_mask = ggml_pad(ctx0, window_mask, 0, n_pad, 0, 0);
+                }
+                window_mask = ggml_cast(ctx0, window_mask, GGML_TYPE_F16);
+            }
+
             // inpL shape: [n_embd, n_patches_x * n_patches_y, batch_size]
             GGML_ASSERT(batch_size == 1);
             inpL = ggml_reshape_2d(ctx0, inpL, n_embd * 4, n_patches_x * n_patches_y * batch_size / 4);