Add ImageProcessor library to ExecuTorch (pytorch#19967)

Summary:

Introduces an ImageProcessor library to ExecuTorch: a small, well-tested
core for turning camera/decoded image buffers into normalized model-input
tensors. This initial implementation is fully portable; performant
platform-specific backends (e.g. Apple Accelerate/Core Image) are added in
follow-up diffs behind the same API.

Given source pixels (BGRA/RGBA bytes or semi-planar YUV planes), the
processor crops/resizes to the configured target and emits a
[1, 3, target_height, target_width] float tensor, normalized per the
configured scheme.

Configuration (`ImageProcessorConfig`):
- target_width / target_height: output spatial size.
- resize_mode: how the source is fit to the target.
- letterbox_anchor: placement of content when letterboxing.
- pad_value: fill value for letterbox padding.
- normalization: how uint8 [0,255] pixels map to floats.
- gpu_min_input_pixels: minimum source pixel count (width * height) at which
  the GPU path is used; smaller inputs run on CPU. Sentinels: 0 = always GPU,
  INT64_MAX = always CPU (defaults to ~1080p, i.e. 1920*1080 + 1). Platform
  backends consult this to pick CPU vs GPU. (Note GPU is always platform optional; but GPU will never be routed when below gpu_min_input_pixels.)

Enum options:
Enum options:
- **ColorFormat** (`BGRA`, `RGBA`): interleaved 8-bit input layout.
- **YUVFormat** (`NV12`, `NV21`): semi-planar YUV input layout.
- **ResizeMode** (`STRETCH`, `LETTERBOX`): `STRETCH` fills the target, ignoring aspect ratio; `LETTERBOX` preserves aspect ratio and pads.
- **LetterboxAnchor** (`CENTER`, `TOP_LEFT`): where padded content is anchored.
- **Orientation** (`UP`): source orientation (upright).
- **Normalization** (`zeroToOne`, `imagenet`): `zeroToOne` scales to [0, 1]; `imagenet` additionally applies per-channel mean/std.


Example:

```
  #include <executorch/extension/image/image_processor.h>

  using namespace executorch::extension::image;

  // Resize to 224x224, letterboxed, with ImageNet normalization.
  ImageProcessorConfig config;
  config.target_width = 224;
  config.target_height = 224;
  config.resize_mode = ResizeMode::LETTERBOX;
  config.normalization = Normalization::imagenet();

  ImageProcessor processor(config);

  // RGBA/BGRA input -> normalized [1, 3, 224, 224] float tensor.
  auto result = processor.process(
      pixels, width, height, /*stride_bytes=*/width * 4, ColorFormat::RGBA);
  if (result.ok()) {
    TensorPtr tensor = std::move(result.get());
    // feed `tensor` to the model...
  }

  // Semi-planar YUV (e.g. NV12 camera frame):
  auto yuv = processor.process_yuv(
      y_plane, y_stride, uv_plane, uv_stride, width, height, YUVFormat::NV12);
```

Differential Revision: D106898421

Author: metascroy

CMakeLists.txt

@@ -864,6 +864,11 @@ if(EXECUTORCH_BUILD_EXTENSION_TENSOR)
   list(APPEND _executorch_extensions extension_tensor)
 endif()
 
+if(EXECUTORCH_BUILD_EXTENSION_IMAGE)
+  add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/extension/image)
+  list(APPEND _executorch_extensions extension_image)
+endif()
+
 if(EXECUTORCH_BUILD_PTHREADPOOL AND EXECUTORCH_BUILD_CPUINFO)
   add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/extension/threadpool)
 endif()

extension/image/BUCK

@@ -0,0 +1,5 @@
+load(":targets.bzl", "define_common_targets")
+
+oncall("executorch")
+
+define_common_targets()

extension/image/CMakeLists.txt

@@ -0,0 +1,45 @@
+# 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.
+
+cmake_minimum_required(VERSION 3.19)
+
+# stb_image_resize: lightweight header-only library used by the resize step in
+# image_processor.cpp.
+include(FetchContent)
+FetchContent_Declare(
+  stb
+  GIT_REPOSITORY https://github.com/nothings/stb.git
+  GIT_TAG f0569113c93ad095470c54bf34a17b36646bbbb5
+)
+FetchContent_MakeAvailable(stb)
+
+add_library(extension_image image_processor_common.cpp image_processor.cpp)
+
+target_include_directories(
+  extension_image PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/../..
+)
+
+# stb_image_resize.h lives under deprecated/ in current stb. Private: only the
+# .cpp uses it, not the installed public headers.
+target_include_directories(
+  extension_image PRIVATE ${stb_SOURCE_DIR} ${stb_SOURCE_DIR}/deprecated
+)
+
+target_link_libraries(extension_image PUBLIC executorch_core extension_tensor)
+
+install(
+  TARGETS extension_image
+  EXPORT ExecuTorchTargets
+  DESTINATION lib
+)
+
+install(FILES image_processor.h image_processor_config.h
+        DESTINATION include/executorch/extension/image
+)
+
+if(BUILD_TESTING)
+  add_subdirectory(test)
+endif()

extension/image/TARGETS

@@ -0,0 +1,5 @@
+load(":targets.bzl", "define_common_targets")
+
+oncall("executorch")
+
+define_common_targets()