Add a mock perf test for Android llama2 tps

Author: huydhn

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

@@ -0,0 +1,57 @@
+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 org.junit.Test;
+import org.junit.runner.RunWith;
+import org.pytorch.executorch.LlamaCallback;
+import org.pytorch.executorch.LlamaModule;
+
+import java.util.ArrayList;
+import java.util.List;
+
+@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 some 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(tps >= EXPECTED_TPS);
+    }
+
+    @Override
+    public void onResult(String result) {
+        results.add(result);
+    }
+
+    @Override
+    public void onStats(float tps) {
+        tokensPerSecond.add(tps);
+    }
+}