add LLM evaluator example

- runs on iOS and macOS
- downloads a model / tokenizer from hugging face
- evaluates the given prompt

Applications/LLMEval/LLMEvalApp.swift

@@ -0,0 +1,12 @@
+// Copyright © 2024 Apple Inc.
+
+import SwiftUI
+
+@main
+struct LLMEvalApp: App {
+    var body: some Scene {
+        WindowGroup {
+            ContentView()
+        }
+    }
+}