Add a mock perf test for llama2 on Android

examples/demo-apps/android/LlamaDemo/app/src/androidTest/java/com/example/executorchllamademo/PerfTest.java

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+/*
+ * 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.
+ */
+
+package com.example.executorchllamademo;
+
+import static junit.framework.TestCase.assertTrue;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+
+import androidx.test.ext.junit.runners.AndroidJUnit4;
+import java.util.ArrayList;
+import java.util.List;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.pytorch.executorch.LlamaCallback;
+import org.pytorch.executorch.LlamaModule;
+
+@RunWith(AndroidJUnit4.class)
+public class PerfTest implements LlamaCallback {
+
+  private static final String RESOURCE_PATH = "/data/local/tmp/llama/";
+  private static final String MODEL_NAME = "xnnpack_llama2.pte";
+  private static final String TOKENIZER_BIN = "tokenizer.bin";
+
+  // From https://github.com/pytorch/executorch/blob/main/examples/models/llama2/README.md
+  private static final Float EXPECTED_TPS = 7.0F;
+
+  private final List<String> results = new ArrayList<>();
+  private final List<Float> tokensPerSecond = new ArrayList<>();
+
+  @Test
+  public void testTokensPerSecond() {
+    String modelPath = RESOURCE_PATH + MODEL_NAME;
+    String tokenizerPath = RESOURCE_PATH + TOKENIZER_BIN;
+    LlamaModule mModule = new LlamaModule(modelPath, tokenizerPath, 0.8f);
+
+    int loadResult = mModule.load();
+    // Check that the model can be load successfully
+    assertEquals(0, loadResult);
+
+    // Run a testing prompt
+    mModule.generate("How do you do! I'm testing llama2 on mobile device", PerfTest.this);
+    assertFalse(tokensPerSecond.isEmpty());
+
+    final Float tps = tokensPerSecond.get(tokensPerSecond.size() - 1);
+    assertTrue(
+        "The observed TPS " + tps + " is less than the expected TPS " + EXPECTED_TPS,
+        tps >= EXPECTED_TPS);
+  }
+
+  @Override
+  public void onResult(String result) {
+    results.add(result);
+  }
+
+  @Override
+  public void onStats(float tps) {
+    tokensPerSecond.add(tps);
+  }
+}