Add Arduino library support for ExecuTorch

examples/arduino/.gitignore

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+# Generated by build_arduino_library.sh — do not check in
+arduino_lib/

examples/arduino/ExecuTorchArduino.h

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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.
+ */
+
+#pragma once
+
+// Arduino's custom <new> header omits <exception>, which breaks
+// std::bad_variant_access in <variant>. Include it first.
+#include <exception>
+
+#define C10_USING_CUSTOM_GENERATED_MACROS
+#define ET_ENABLE_DEPRECATED_CONSTANT_BUFFER 0
+#define FLATBUFFERS_MAX_ALIGNMENT 1024
+
+#include <executorch/runtime/platform/runtime.h>
+#include <executorch/runtime/executor/program.h>
+#include <executorch/runtime/executor/method.h>
+#include <executorch/runtime/core/memory_allocator.h>
+#include <executorch/runtime/executor/method_meta.h>
+#include <executorch/extension/data_loader/buffer_data_loader.h>

examples/arduino/README.md

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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.
+--->
+
+# ExecuTorch Arduino Library
+
+Run PyTorch models on Arduino microcontrollers using ExecuTorch.
+
+This directory contains everything needed to package ExecuTorch as an
+Arduino library. A build script vendors the runtime sources from this
+repository into a self-contained library that Arduino users install
+through the Library Manager or by copying into their libraries folder.
+
+## How It Works
+
+```
+PyTorch Model ──► torch.export ──► .pte file ──► model.h (C array)
+                                                      │
+                                          Arduino Sketch (.ino)
+                                          #include <ExecuTorchArduino.h>
+                                          #include "model.h"
+                                                      │
+                                          arduino-cli compile ──► Upload ──► Runs on board
+```
+
+### The three pieces
+
+1. **The library** (`arduino_lib/ExecuTorchArduino/`) — the ExecuTorch
+   runtime, CMSIS-NN kernels, and portable ops packaged for the Arduino
+   build system.  Generated by `build_arduino_library.sh`; not checked in.
+
+2. **The model** (`model.h`) — a `.pte` file converted to a C byte array.
+   Each user brings their own model, exported from PyTorch with the
+   Cortex-M backend.
+
+3. **The sketch** (`.ino`) — a standard Arduino program that loads the
+   model, feeds it input, and reads the output.  Uses the native
+   ExecuTorch C++ API (`Program::load`, `Method::execute`, etc.).
+
+## Supported Boards
+
+| Board | MCU | Status |
+|-------|-----|--------|
+| Arduino Uno Q | STM32U585 (Cortex-M33) | Tested |
+| Arduino Nano 33 BLE | nRF52840 (Cortex-M4F) | Untested |
+| Arduino Giga R1 WiFi | STM32H747 (Cortex-M7) | Untested |
+| Arduino Portenta H7 | STM32H747 (Cortex-M7) | Untested |
+
+CMSIS-NN accelerated ops work on any board with an ARM Cortex-M processor
+with DSP extensions. Portable ops work on any architecture.
+
+## Quick Start
+
+### 1. Build the Arduino library
+
+```bash
+cd examples/arduino
+./build_arduino_library.sh
+```
+
+This copies the required ExecuTorch sources from the repository into
+`arduino_lib/ExecuTorchArduino/`, ready for Arduino.
+
+### 2. Install the library
+
+Copy the generated library into your Arduino libraries folder:
+
+```bash
+cp -r arduino_lib/ExecuTorchArduino ~/Arduino/libraries/
+```
+
+Or with `arduino-cli`:
+
+```bash
+arduino-cli lib install --zip-path arduino_lib/ExecuTorchArduino
+```
+
+### 3. Export a model
+
+Export a PyTorch model to `.pte` format with Cortex-M quantization, then
+convert to a C header:
+
+```bash
+python export_model.py --model my_model.pt --target cortex-m33 --output model.h
+```
+
+Or use one of the pre-exported models in the `examples/` directory.
+
+### 4. Write a sketch
+
+```cpp
+#include <ExecuTorchArduino.h>
+#include "model.h"
+
+using executorch::extension::BufferDataLoader;
+using executorch::runtime::Error;
+using executorch::runtime::HierarchicalAllocator;
+using executorch::runtime::MemoryAllocator;
+using executorch::runtime::MemoryManager;
+using executorch::runtime::Method;
+using executorch::runtime::MethodMeta;
+using executorch::runtime::Program;
+using executorch::runtime::Result;
+using executorch::runtime::Span;
+
+alignas(16) uint8_t method_pool[64 * 1024];
+alignas(16) uint8_t temp_pool[8 * 1024];
+
+void setup() {
+  Serial.begin(115200);
+  delay(2000);
+
+  executorch::runtime::runtime_init();
+
+  auto loader = BufferDataLoader(model_pte, model_pte_size);
+  Result<Program> program = Program::load(&loader);
+  if (!program.ok()) {
+    Serial.println("Failed to load program");
+    return;
+  }
+
+  // ... load method, set inputs, execute, read outputs
+  // See examples/ for complete working sketches.
+}
+
+void loop() {
+  // Run inference periodically
+  delay(2000);
+}
+```
+
+The sketch uses the **native ExecuTorch C++ API** — the same API used on
+Linux, Android, and bare-metal targets.  No wrapper layer, no
+Arduino-specific abstractions.
+
+### 5. Compile and upload
+
+```bash
+arduino-cli compile --fqbn arduino:zephyr:unoq MySketch
+arduino-cli upload  --fqbn arduino:zephyr:unoq -p /dev/ttyUSB0 MySketch
+```
+
+## What is inside the library
+
+The `build_arduino_library.sh` script assembles these components from
+the ExecuTorch repository:
+
+| Component | Source in repo | Purpose |
+|-----------|---------------|---------|
+| ET Runtime | `runtime/executor/`, `runtime/core/`, `runtime/kernel/`, `runtime/platform/` | Model loading, memory management, op dispatch |
+| Portable Ops | `kernels/portable/` | Software op implementations (any CPU) |
+| Cortex-M Ops | `backends/cortex_m/ops/` | CMSIS-NN accelerated int8 ops |
+| CMSIS-NN | fetched by cmake / Zephyr module | ARM's optimized DSP kernels |
+| flatcc | `third-party/flatcc/` | .pte file parsing |
+| flatbuffers | `third-party/flatbuffers/` | Schema headers |
+| c10 | `runtime/core/portable_type/c10/` | Core type definitions |
+
+The library uses no external dependencies beyond what the Arduino board
+core provides.
+
+## Arduino-specific patches
+
+The build script applies these patches to make ExecuTorch compile under
+Arduino's build system:
+
+1. **`#include <exception>` before `<variant>`** — Arduino's custom
+   `<new>` header omits `<exception>`, breaking `std::bad_variant_access`.
+
+2. **`cmake_macros.h` stub** — c10/torch headers expect a cmake-generated
+   file.  The stub defines `C10_USING_CUSTOM_GENERATED_MACROS`.
+
+3. **`platform_stubs.c`** — provides `_Exit()` and `fprintf()` for the
+   LLEXT environment on boards that lack them.
+
+## Development
+
+### Updating the library
+
+After modifying ExecuTorch sources, regenerate the library:
+
+```bash
+./build_arduino_library.sh        # rebuild
+./build_arduino_library.sh --clean  # remove generated output
+```
+
+### Testing
+
+```bash
+arduino-cli compile --fqbn arduino:zephyr:unoq examples/HelloExecuTorch
+arduino-cli upload  --fqbn arduino:zephyr:unoq -p /dev/ttyUSB0 examples/HelloExecuTorch
+arduino-cli monitor -p /dev/ttyUSB0 --config baudrate=115200
+```
+
+### Publishing to Arduino Library Manager
+
+The library is published by adding its repository URL to the
+[Arduino Library Registry](https://github.com/arduino/library-registry).
+After the initial registration, new git tags are picked up automatically.
+
+## Build Validation
+
+Tested on Arduino Uno Q (STM32U585, Cortex-M33 @ 160 MHz):
+
+- **390+ source files** compile with zero errors
+- **Flash**: 106 KB used (13% of 786 KB)
+- **RAM**: 91 KB used (69% of 131 KB)
+- **Model**: DS-CNN keyword spotting, int8 quantized via CMSIS-NN, 52.6 KB