Update metadata.json: add missing <image> placeholder to prompt_template

metadata.json

@@ -10,7 +10,7 @@
         "single_image_start"
       ],
       "task_type": "vqa",
-      "prompt_template": "{{ question }}",
+      "prompt_template": "<image>{{ question }}",
       "mapping_from_source": {
         "media": {
           "from": "images",