[V1] Scatter and gather placeholders in the model runner (#15712)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>
Signed-off-by: mgoin <mgoin64@gmail.com>
Signed-off-by: Roger Wang <ywang@roblox.com>
Co-authored-by: mgoin <mgoin64@gmail.com>
Co-authored-by: Roger Wang <ywang@roblox.com>

tests/models/multimodal/processing/test_llava_onevision.py

@@ -92,8 +92,8 @@ def _validate_image_prompt_replacements_one(
 
         first_placeholder = image_placeholders[0]
 
-        assert first_placeholder["offset"] == 0
-        assert first_placeholder["length"] == len(
+        assert first_placeholder.offset == 0
+        assert first_placeholder.length == len(
             processed_inputs["prompt_token_ids"]) // num_imgs
     except Exception as exc:
         failed_size_excs.append((image_size, exc))