[onnx] Add torch-mlir-import-onnx native port as an optional tool/library. (#2694)

As noted in the plan when this work started, we need to produce an ORT
EP plugin for a downstream project, and this will necessitate a C-based
ONNX importer (as opposed to the existing Python one). Because this
comes with dependencies that we do not want to impart on various
projects, this is optional in torch-mlir. It is also factored so that it
can be used as standalone sources in downstreams that need it. Since it
only depends on public C APIs on the MLIR side, this will make build
coupling a lot better (since a C++ dep is not needed on the compiler and
it is trivial to dynamically load).

Our original plan was just to maintain this fork off to the side in our
ORT plugin, but once work started, it seemed better to write it clean
and contribute it upstream for anyone to use. We expect that for non-ORT
use, the Python importer will have better ergonomics for most folks.

I will follow-up with a test suite refactor so that we can drive the
Python or C importer.

This is a relatively mechanical port from Python to C, borrowing some
scaffolding from the old JitIR importer. It does attempt to lay some
groundwork for external data, which will need to be implemented on the
Python side as well.

Author: stellaraccident

CMakeLists.txt

@@ -51,6 +51,8 @@ option(TORCH_MLIR_ENABLE_PYTORCH_EXTENSIONS "Enables PyTorch native extension fe
 cmake_dependent_option(TORCH_MLIR_ENABLE_JIT_IR_IMPORTER "Enables JIT IR Importer" ON TORCH_MLIR_ENABLE_PYTORCH_EXTENSIONS OFF)
 cmake_dependent_option(TORCH_MLIR_ENABLE_LTC "Enables LTC backend" OFF TORCH_MLIR_ENABLE_PYTORCH_EXTENSIONS OFF)
 
+option(TORCH_MLIR_ENABLE_ONNX_C_IMPORTER "Enables the ONNX C importer" OFF)
+
 #-------------------------------------------------------------------------------
 # Configure out-of-tree vs in-tree build
 #-------------------------------------------------------------------------------

projects/CMakeLists.txt

@@ -1,5 +1,9 @@
 include(AddMLIRPython)
 
+if(TORCH_MLIR_ENABLE_ONNX_C_IMPORTER)
+  add_subdirectory(onnx_c_importer)
+endif()
+
 ################################################################################
 # PyTorch
 # Configure PyTorch if we have any features enabled which require it.

projects/onnx_c_importer/CMakeLists.txt

@@ -0,0 +1,44 @@
+message(STATUS "Enabling onnx_c_importer...")
+
+include(FetchContent)
+
+find_package(Protobuf)
+if(NOT Protobuf_FOUND)
+    message(FATAL_ERROR 
+        "In order to build C ONNX support, the Protobuf package must be installed "
+        "on the system. Without this ONNX will attempt to build it in the project "
+        "and the dependent ABSEIL build system is incompatible. "
+        "On Ubuntu, install with: "
+        "apt install libprotobuf-dev protobuf-compiler\n\n"
+        "(or this entire component can be disabled with "
+        "-DTORCH_MLIR_ENABLE_ONNX_C_IMPORTER=OFF)")
+endif()
+
+option(ONNX_DISABLE_EXCEPTIONS "For compatibility with LLVM build" ON)
+
+FetchContent_Declare(
+    onnx
+    EXCLUDE_FROM_ALL
+    GIT_REPOSITORY https://github.com/onnx/onnx.git
+    GIT_TAG v1.15.0
+    GIT_SHALLOW ON
+    GIT_PROGRESS ON
+)
+FetchContent_MakeAvailable(onnx)
+
+add_llvm_executable(
+    torch-mlir-import-onnx
+    PARTIAL_SOURCES_INTENDED
+
+    import-onnx-main.cpp
+    OnnxImporter.h
+    OnnxImporter.cpp
+)
+
+target_link_libraries(
+    torch-mlir-import-onnx
+    LLVMSupport
+    MLIRCAPIIR
+    TorchMLIRCAPI
+    onnx
+)