mtmd : fix batch_view for m-rope (#13397)

* mtmd : fix batch_view for m-rope

* nits : fix comment

Author: ngxson

tools/mtmd/mtmd.cpp

@@ -554,14 +554,19 @@ struct decode_embd_batch {
     llama_batch get_view(int offset, int n_tokens) {
         llama_pos * pos_ptr;
         pos_view.clear();
-        pos_view.resize(n_tokens * n_pos_per_embd);
+        pos_view.reserve(n_tokens * n_pos_per_embd);
         if (n_pos_per_embd > 1) {
             // mrope
             // for example, with layout of src: 1234...1234...1234...1234...
             //       offset 2 will give us dst: 34...34...34...34...
             for (int i = 0; i < n_pos_per_embd; i++) {
-                auto src = pos.begin() + i * batch.n_tokens + offset;
-                pos_view.insert(pos_view.end(), src, src + n_tokens);
+                // assume n_tokens is less than or equal to batch.n_tokens
+                // batch.n_tokens is number of **total** tokens
+                // n_tokens is number of viewed token
+                size_t src_idx = i * batch.n_tokens + offset;
+                pos_view.insert(pos_view.end(),
+                    pos.data() + src_idx,
+                    pos.data() + src_idx + n_tokens);
             }
             pos_ptr = pos_view.data();
         } else {