Add prompt constants file (pytorch#10618)

Author: jackzhxng

examples/demo-apps/apple_ios/LLaMA/LLaMA/Application/Constants.swift

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import Foundation
+
+public enum Constants {
+    public static let qwen3PromptTemplate = """
+<|im_start|>system
+You are a helpful assistant.
+<|im_end|>
+<|im_start|>user
+%@<|im_end|>
+<|im_start|>assistant
+<think>
+
+</think>
+
+
+
+"""
+    
+    public static let llama3PromptTemplate = "<|begin_of_text|><|start_header_id|>user<|end_header_id|>%@<|eot_id|><|start_header_id|>assistant<|end_header_id|>"
+}