mtmd, model: allow skip build_vit() (#24077)

* add model

* nits

tools/mtmd/models/gemma4ua.cpp

@@ -0,0 +1,19 @@
+#include "models.h"
+#include <cmath>
+
+ggml_cgraph * clip_graph_gemma4ua::build() {
+    ggml_tensor * inp = build_inp_raw(1);
+
+    auto cur = ggml_cont(ctx0, ggml_permute(ctx0, inp, 1, 0, 2, 3));
+
+    // Gemma4UnifiedMultimodalEmbedder
+    {
+        // embedding_pre_projection_norm
+        cur = ggml_rms_norm(ctx0, cur, hparams.eps);
+        cur = build_mm(model.mm_input_proj_w, cur);
+        cb(cur, "projected", -1);
+    }
+
+    ggml_build_forward_expand(gf, cur);
+    return gf;
+}