Add a mock perf test for llama2 on Android (#2963)

Summary:
I'm trying to setup a simple perf test when running llama2 on Android.  It's naively sent a prompt and record the TPS.  Open for comment about the test here before setting this up on CI.

### Testing

Copy the exported model and the tokenizer as usual, then cd to the app and run `./gradlew :app:connectAndroidTest`.  The test will fail if the model is failed to load or if the TPS is lower than 7 as measure by https://github.com/pytorch/executorch/tree/main/examples/models/llama2

Pull Request resolved: #2963

Reviewed By: kirklandsign

Differential Revision: D55951637

Pulled By: huydhn

fbshipit-source-id: 34c189aefd7e31514fcf49103352ef3cf8e5b2c9

Author: huydhn

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

@@ -0,0 +1,65 @@
+/*
+ * 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);
+  }
+}