LlamaDemo Android add Qwen 3 prompt format (#10624)

Format the raw prompt

Author: kirklandsign

examples/demo-apps/android/LlamaDemo/app/src/main/java/com/example/executorchllamademo/ModelType.java

@@ -14,4 +14,5 @@ public enum ModelType {
   LLAMA_3_2,
   LLAVA_1_5,
   LLAMA_GUARD_3,
+  QWEN_3,
 }

examples/demo-apps/android/LlamaDemo/app/src/main/java/com/example/executorchllamademo/ModelUtils.java

@@ -29,6 +29,7 @@ public static int getModelCategory(ModelType modelType, BackendType backendType)
         case LLAMA_3:
         case LLAMA_3_1:
         case LLAMA_3_2:
+        case QWEN_3:
         default:
           return TEXT_MODEL;
       }

examples/demo-apps/android/LlamaDemo/app/src/main/java/com/example/executorchllamademo/PromptFormat.java

@@ -25,6 +25,8 @@ public static String getSystemPromptTemplate(ModelType modelType) {
             + "<|eot_id|>";
       case LLAVA_1_5:
         return "USER: ";
+      case QWEN_3:
+        return "<|im_start|>system\n" + "You are a helpful assistant.\n" + "<|im_end|>\n";
       default:
         return SYSTEM_PLACEHOLDER;
     }
@@ -42,6 +44,14 @@ public static String getUserPromptTemplate(ModelType modelType) {
             + "<|start_header_id|>assistant<|end_header_id|>";
 
       case LLAVA_1_5:
+      case QWEN_3:
+        return "<|im_start|>user\n"
+            + USER_PLACEHOLDER
+            + "<|im_end|>\n"
+            + "<|im_start|>assistant\n"
+            + "<think>\n"
+            + "\n"
+            + "</think>\n\n\n";
       default:
         return USER_PLACEHOLDER;
     }
@@ -69,6 +79,8 @@ public static String getStopToken(ModelType modelType) {
         return "<|eot_id|>";
       case LLAVA_1_5:
         return "</s>";
+      case QWEN_3:
+        return "<|endoftext|>";
       default:
         return "";
     }