[Core] Move multimodal placeholder from chat utils to model definition (#20355)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

vllm/model_executor/models/glm4v.py

@@ -540,6 +540,13 @@ class GLM4VForCausalLM(ChatGLMBaseModel, SupportsLoRA, SupportsPP,
             connector="transformer.vision.linear_proj",
             tower_model="transformer.vision.transformer")
 
+    @classmethod
+    def get_placeholder_str(cls, modality: str, i: int) -> Optional[str]:
+        if modality.startswith("image"):
+            return "<|begin_of_image|><|endoftext|><|end_of_image|>"
+
+        raise ValueError("Only image modality is supported")
+
     def __init__(
         self,
         *,