Add a readme file with build instructions. (#586)

Summary:
Pull Request resolved: #586

.

Reviewed By: cccclai

Differential Revision: D49866403

fbshipit-source-id: 1012457a3c819854144c8cf0c2aaa78edad217cc

Author: shoumikhin

examples/apple/ios/README.md

@@ -0,0 +1,163 @@
+# iOS Demo App: Executorch Setup
+
+This guide explains how to setup Executorch for iOS using a demo app. The app
+employs a MobileNet v3 model (exported to Executorch) to process live camera
+images.
+
+## Pre-setup
+
+1. Install Xcode and Command Line Tools:
+
+   ```bash
+   xcode-select --install
+   ```
+
+2. Install `buck2` binary (using MacOS Apple Silicon build as example):
+
+   ```bash
+   curl -L -O https://github.com/facebook/buck2/releases/download/2023-07-18/buck2-aarch64-apple-darwin.zst
+   pip3 install zstd
+   zstd -cdq buck2-aarch64-apple-darwin.zst > /tmp/buck2 && chmod +x /tmp/buck2
+   ```
+
+3. Install [Cmake](cmake.org/download) and link it in a system directory or
+   `$PATH`:
+
+   ```bash
+   ln -s /Applications/CMake.app/Contents/bin/cmake /usr/bin/cmake
+   ```
+
+4. Clone Executorch repository and update submodules:
+
+   ```bash
+   git clone https://github.com/pytorch/executorch.git
+   cd executorch
+   git submodule sync
+   git submodule update --init
+   ```
+
+5. Verify Python 3.10+ (standard since MacOS 13.5) and `pip3` installation:
+
+   ```bash
+   which python3 pip
+   python3 --version
+   ```
+
+6. Install PyTorch dependencies:
+
+   ```bash
+   ./install_requirements.sh
+   ```
+
+## Flatbuffers Compiler Setup
+
+Run the following in the `flatbuffers` directory:
+
+```bash
+cd third-party/flatbuffers
+rm -rf cmake-out && mkdir cmake-out && cd cmake-out
+cmake .. && cmake --build . --target flatc
+cd ../../..
+```
+
+## Executorch Configuration
+
+Configure the libraries for iOS:
+
+```bash
+rm -rf cmake-out && mkdir cmake-out && cd cmake-out
+cmake .. -G Xcode \
+    -DCMAKE_TOOLCHAIN_FILE=../third-party/pytorch/cmake/iOS.cmake \
+    -DBUCK2=/tmp/buck2 \
+    -DPYTHON_EXECUTABLE=$(which python3) \
+    -DFLATC_EXECUTABLE=$(realpath ../third-party/flatbuffers/cmake-out/flatc) \
+    -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON \
+    -DCMAKE_ARCHIVE_OUTPUT_DIRECTORY=$(pwd)
+```
+
+Append `-DIOS_PLATFORM=SIMULATOR` for Simulator configuration to build libraries
+for `x86` architecture instead of `arm64`, which is the default.
+
+Append `-DEXECUTORCH_BUILD_XNNPACK=ON` to build XNNPACK backend libraries.
+
+## Building and Copying Libraries
+
+1. Build the libraries:
+
+```bash
+cmake --build . --config Release
+```
+
+2. Copy the libraries to the appropriate location to link against them:
+
+Navigate to the build artifacts directory:
+
+```bash
+cd Release
+mkdir -p ../../examples/apple/ios/ExecuTorchDemo/ExecuTorchDemo/Frameworks/executorch/
+```
+
+Copy the core libraries:
+
+```bash
+cp libexecutorch.a \
+   libextension_data_loader.a \
+   ../../examples/apple/ios/ExecuTorchDemo/ExecuTorchDemo/Frameworks/executorch/
+```
+
+For Portable or Benchmark operators, copy additional libraries:
+
+```bash
+cp libportable_kernels.a \
+   libportable_ops_lib.a \
+   ../../examples/apple/ios/ExecuTorchDemo/ExecuTorchDemo/Frameworks/executorch/
+```
+
+For XNNPACK backend, copy additional libraries:
+
+```bash
+cp libclog.a \
+   libcpuinfo.a \
+   libpthreadpool.a \
+   libxnnpack_backend.a \
+   libXNNPACK.a \
+   ../../examples/apple/ios/ExecuTorchDemo/ExecuTorchDemo/Frameworks/executorch/
+```
+
+Then return to the `executorch` directory:
+
+```bash
+cd ../..
+```
+
+## Model Download and Bundling
+
+1. Download MobileNet model labels and bundle them with the app:
+
+```bash
+mkdir -p examples/apple/ios/ExecuTorchDemo/ExecuTorchDemo/Resources/Models/MobileNet/
+curl https://github.com/raw/pytorch/hub/master/imagenet_classes.txt -o examples/apple/ios/ExecuTorchDemo/ExecuTorchDemo/Resources/Models/MobileNet/imagenet_classes.txt
+```
+
+2. Export a MobileNet v3 model backed with XNNPACK delegate and bundle it with
+   the app:
+
+```bash
+export FLATC_EXECUTABLE=$(realpath third-party/flatbuffers/cmake-out/flatc)
+python3 -m examples.export.export_example --model_name="mv3"
+python3 -m examples.backend.xnnpack_examples --model_name="mv3" --delegate
+cp mv3.pte mv3_xnnpack_fp32.pte examples/apple/ios/ExecuTorchDemo/ExecuTorchDemo/Resources/Models/MobileNet/
+```
+
+## Final Steps
+
+1. Open the project with Xcode:
+
+```bash
+open executorch/examples/apple/ios/ExecuTorchDemo/ExecuTorchDemo.xcodeproj
+```
+
+2. Set the Header Search Paths for `MobileNetClassifier` target to the directory
+   containing the `executorch` folder.
+
+3. Run the app (Cmd + R) and tests (Cmd + U).