[cmake] Place export target file in the correct directory

[ashay]

Description

Prior to this patch, the ONNXConfig.cmake script (generated from the
ONNXConfig.cmake.in file) tries to read the export target file
(ONNXTargets.cmake) from the same directory where ONNXConfig.cmake is
located, but the two files are not in the same directory. Specifically,
CMake places the export target file (ONNXTargets.cmake) in a directory
whose name contains the build hash, whereas the ONNXConfig.cmake script
is placed in the ${PROJECT_BINARY_DIR}/ONNXConfig.cmake, as specified in
the top-level CMakeLists.txt directory.

This patch reconciles the difference by explicitly instructing CMake to
place the export target file (ONNXTargets.cmake) in the same directory
as the ONNXConfig.cmake script during the build step.

This patch also makes two ancillary changes. First, it adds the
ONNX:: namespace to the target names, so that targets built by this
project don't interfere with targets built by other projects. Second,
this patch adds Protobuf to the list of packages that consumers of ONNX
must have available, since protobuf libraries are linked publicly while
building ONNX.

🤖 Generated by Copilot at 5173943

Summary

🛠️🚀🌐

This pull request enhances the ONNX CMake configuration to make the Protobuf library more accessible and consistent for the ONNX library and its users. It modifies the CMakeLists.txt and cmake/ONNXConfig.cmake.in files to set and export the relevant Protobuf variables and commands.

Sing, O Muse, of the valiant ONNX project, the source of many models
That with CMake configures its code, and to other projects unfolds
Its mighty Protobuf dependency, that it finds and exports with care
Like Zeus who wields his thunderbolt, and his power with the gods shares

Walkthrough

• Add find_package(Protobuf REQUIRED) command and set CMAKE_PREFIX_PATH and Protobuf_INCLUDE_DIR variables to make Protobuf library findable by ONNXConfig.cmake file (link)
• Set PROTOBUF_DIR and PROTOBUF_INCLUDE_DIR variables to Protobuf installation prefix or binary and source directories depending on whether Protobuf was fetched and built by ONNX or not (link, link)
• Add NAMESPACE ONNX:: option to install(EXPORT ONNXTargets ...) and export(EXPORT ONNXTargets ...) commands to follow CMake best practices and avoid name clashes (link)
• Add conditional export(EXPORT ONNXTargets ...) command to workaround Protobuf export issue when it is built as part of ONNX (link)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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[jcwchen]

Please ignore the failure from Windows release CI, which will be fixed by #5678.