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

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

vllm/model_executor/models/kimi_vl.py

@@ -264,6 +264,13 @@ class KimiVLMultiModalProcessor(BaseMultiModalProcessor[KimiVLProcessingInfo]):
                                         dummy_inputs=KimiVLDummyInputsBuilder)
 class KimiVLForConditionalGeneration(nn.Module, SupportsMultiModal):
 
+    @classmethod
+    def get_placeholder_str(cls, modality: str, i: int) -> Optional[str]:
+        if modality.startswith("image"):
+            return "<|media_start|>image<|media_content|><|media_pad|><|media_end|>"
+
+        raise ValueError("Only image modality is supported")
+
     def __init__(
         self,
         vllm_config: VllmConfig,