[Model] Add support for Gemma 3 (#14660)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>
Signed-off-by: Roger Wang <ywang@roblox.com>
Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>
Co-authored-by: Roger Wang <ywang@roblox.com>
Co-authored-by: DarkLight1337 <tlleungac@connect.ust.hk>

vllm/entrypoints/chat_utils.py

@@ -433,6 +433,8 @@ class BaseMultiModalItemTracker(ABC, Generic[_T]):
                 return "<image>"
             if model_type == "aria":
                 return "<|fim_prefix|><|img|><|fim_suffix|>"
+            if model_type == "gemma3":
+                return "<start_of_image>"
 
             raise TypeError(f"Unknown {modality} model type: {model_type}")
         elif modality == "audio":