Fix demo app workflow and tutorial (#1979)

Summary:
Now we use sources from extension/android, and we need to update the instructions for Android demo app.

Note: We have two ad-hoc solutions

1. For Java, so far we are copying sources; we should use package instead.
2. For JNI, we are building from top-level CMakeLists.txt; we should build from extension/android and `find_package()` for deps.

We will address these two issues in follow-up PRs.

Pull Request resolved: #1979

Reviewed By: mcr229

Differential Revision: D53791261

Pulled By: kirklandsign

fbshipit-source-id: 73e48f1cc3c444426b8b3252fc48762aa182cb98

Author: kirklandsign

CMakeLists.txt

@@ -305,9 +305,9 @@ if(EXECUTORCH_BUILD_EXECUTOR_RUNNER)
   target_compile_options(executor_runner PUBLIC ${_common_compile_options})
 endif()
 
-# Add Android demo app JNI subdirectory
-if(EXECUTORCH_BUILD_ANDROID_DEMO_APP_JNI)
-  add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/examples/demo-apps/android/jni)
+# Add Android JNI subdirectory
+if(EXECUTORCH_BUILD_ANDROID_JNI)
+  add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/extension/android)
 endif()
 
 if(EXECUTORCH_BUILD_SDK)

examples/demo-apps/android/ExecuTorchDemo/README.md

@@ -62,27 +62,40 @@ cp deeplab_v3/dlv3_qnn.pte examples/demo-apps/android/ExecuTorchDemo/app/src/mai
 
 We build the required ExecuTorch runtime library to run the model.
 
+#### Java helper classes
+
+Note: This is an ad-hoc solution. We will publish a formal Java package when it is ready. However, for now we need to copy sources from extension/android/src/main/java/org/pytorch/executorch.
+
+```
+mkdir -p examples/demo-apps/android/ExecuTorchDemo/app/src/main/java/com/example/executorchdemo/executor
+cp extension/android/src/main/java/org/pytorch/executorch/*.java examples/demo-apps/android/ExecuTorchDemo/app/src/main/java/com/example/executorchdemo/executor
+```
+
 #### XNNPACK
 
 1. Configure the CMake target for the library with XNNPACK backend:
 
 ```bash
 export ANDROID_NDK=<path-to-android-ndk>
+export BUCK2=/tmp/buck2 # Or your buck path
 
 rm -rf cmake-out && mkdir cmake-out && cd cmake-out
-cmake .. \
-    -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK/build/cmake/android.toolchain.cmake \
-    -DANDROID_ABI=arm64-v8a \
-    -DBUCK2=/tmp/buck2 \
-    -DEXECUTORCH_BUILD_ANDROID_DEMO_APP_JNI=ON \
-    -DEXECUTORCH_BUILD_XNNPACK=ON \
-    -DEXECUTORCH_BUILD_FLATC=OFF \
-    -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON
+cmake .. -DCMAKE_INSTALL_PREFIX=cmake-out \
+        -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK/build/cmake/android.toolchain.cmake \
+        -DANDROID_ABI=arm64-v8a \
+        -DBUCK2=$BUCK \
+        -DEXECUTORCH_BUILD_XNNPACK=ON \
+        -DEXECUTORCH_BUILD_FLATC=OFF \
+        -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON \
+        -DFLATC_EXECUTABLE=$FLATC_EXECUTABLE \
+        -DEXECUTORCH_BUILD_ANDROID_JNI=ON \
+        -DEXECUTORCH_BUILD_EXTENSION_MODULE=ON \
+        -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON
 ```
 
-When we set `EXECUTORCH_BUILD_XNNPACK=ON`, we will build the target [`xnn_executor_runner_lib`](https://github.com/pytorch/executorch/blob/main/backends/xnnpack/CMakeLists.txt) which in turn is linked into libexecutorchdemo via [CMake](https://github.com/pytorch/executorch/blob/main/examples/demo-apps/android/jni/CMakeLists.txt).
+When we set `EXECUTORCH_BUILD_XNNPACK=ON`, we will build the target [`xnn_executor_runner_lib`](https://github.com/pytorch/executorch/blob/main/backends/xnnpack/CMakeLists.txt) which in turn is linked into libexecutorch_jni via [CMake](https://github.com/pytorch/executorch/blob/main/examples/demo-apps/android/jni/CMakeLists.txt).
 
-`libexecutorchdemo.so` wraps up the required XNNPACK Backend runtime library from `xnn_executor_runner_lib`, and adds an additional JNI layer using fbjni. This is later exposed to Java app.
+`libexecutorch_jni.so` wraps up the required XNNPACK Backend runtime library from `xnn_executor_runner_lib`, and adds an additional JNI layer using fbjni. This is later exposed to Java app.
 
 2. Build the libraries:
 
@@ -110,7 +123,7 @@ cmake .. \
     -DQNN_SDK_ROOT=$QNN_SDK \
     -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON
 ```
-Similar to the XNNPACK library, with this setup, we compile `libexecutorchdemo.so` but it adds an additional static library `qnn_executorch_backend` which wraps up Qualcomm HTP runtime library and registers the Qualcomm HTP backend. This is later exposed to Java app.
+Similar to the XNNPACK library, with this setup, we compile `libexecutorch_jni.so` but it adds an additional static library `qnn_executorch_backend` which wraps up Qualcomm HTP runtime library and registers the Qualcomm HTP backend. This is later exposed to Java app.
 
 `qnn_executorch_backend` is built when we turn on CMake option `EXECUTORCH_BUILD_QNN`. It will include the [CMakeLists.txt](https://github.com/pytorch/executorch/blob/main/backends/qualcomm/CMakeLists.txt) from backends/qualcomm where we `add_library(qnn_executorch_backend STATIC)`.
 
@@ -131,8 +144,8 @@ mkdir -p ../examples/demo-apps/android/ExecuTorchDemo/app/src/main/jniLibs/arm64
 Copy the core libraries:
 
 ```bash
-cp ./examples/demo-apps/android/jni/libexecutorchdemo.so \
-   ../examples/demo-apps/android/ExecuTorchDemo/app/src/main/jniLibs/arm64-v8a
+cp ./examples/demo-apps/android/jni/libexecutorch_jni.so \
+   ../examples/demo-apps/android/ExecuTorchDemo/app/src/main/jniLibs/arm64-v8a/libexecutorch.so
 ```
 
 This allows the Android app to load ExecuTorch runtime with XNNPACK backend as a JNI library. Later, this shared library will be loaded by `NativePeer.java` in Java code.
@@ -153,8 +166,8 @@ cp ${QNN_SDK_ROOT}/lib/aarch64-android/libQnnHtp.so ${QNN_SDK_ROOT}/lib/aarch64-
 Copy the core libraries:
 
 ```bash
-cp ./examples/demo-apps/android/jni/libexecutorchdemo.so \
-   ../examples/demo-apps/android/ExecuTorchDemo/app/src/main/jniLibs/arm64-v8a
+cp ./examples/demo-apps/android/jni/libexecutorch_jni.so \
+   ../examples/demo-apps/android/ExecuTorchDemo/app/src/main/jniLibs/arm64-v8a/libexecutorch.so
 ```
 
 ## Running the App