[onnx] Enable --iree-input-type=onnx (iree-org#15995)

Includes a one-line fix to torch-mlir (already landed) that fixes an
unused variable warning.

With this, the following now works (for a model.onnx from the test
suite):

```
python -m iree.compiler.tools.import_onnx ~/tmp/model.onnx | ./tools/iree-compile --iree-input-type=onnx --iree-hal-target-backends=vmvx - -o /dev/null
```

We've still got a way to go on the conversions before there is enough
coverage to be generally usable.

compiler/plugins/input/Torch/torch-iree/CMakeLists.txt

@@ -20,6 +20,7 @@ iree_cc_library(
     torch-mlir::TorchConversionDialectIR
     torch-mlir::TorchDialectIR
     torch-mlir::TorchDialectPasses
+    torch-mlir::TorchOnnxToTorchPasses
     torch-mlir::ConversionPasses
     torch-mlir-dialects::TMTensorDialectIR
   PUBLIC

compiler/plugins/input/Torch/torch-iree/PluginRegistration.cpp

@@ -11,6 +11,7 @@
 #include "torch-iree/InputConversion/Passes.h"
 #include "torch-mlir-dialects/Dialect/TMTensor/IR/TMTensorDialect.h"
 #include "torch-mlir/Conversion/Passes.h"
+#include "torch-mlir/Conversion/TorchOnnxToTorch/Passes.h"
 #include "torch-mlir/Dialect/Torch/IR/TorchDialect.h"
 #include "torch-mlir/Dialect/TorchConversion/IR/TorchConversionDialect.h"
 #include "torch-mlir/Dialect/TorchConversion/Transforms/Passes.h"
@@ -39,6 +40,7 @@ struct TorchSession
     mlir::torch::registerTorchPasses();
     mlir::torch::registerTorchConversionPasses();
     mlir::torch::registerConversionPasses();
+    mlir::torch::onnx_c::registerTorchOnnxToTorchPasses();
     TorchInput::registerTMTensorConversionPasses();
   }
 
@@ -51,14 +53,21 @@ struct TorchSession
 
   bool extendCustomInputConversionPassPipeline(
       OpPassManager &passManager, std::string_view typeMnemonic) override {
-    if (typeMnemonic == "torch") {
+    if (typeMnemonic == "onnx") {
+      // ONNX input is a pre-processing step to torch.
+      passManager.addNestedPass<func::FuncOp>(
+          mlir::torch::onnx_c::createTorchOnnxToTorchPass());
+    }
+
+    if (typeMnemonic == "torch" || typeMnemonic == "onnx") {
       TorchInput::TorchToIREELoweringPipelineOptions torchOptions;
       torchOptions.strictSymbolicShapes = options.strictSymbolicShapes;
       TorchInput::createTorchToIREEPipeline(passManager, torchOptions);
       passManager.addNestedPass<func::FuncOp>(
           TorchInput::createConvertTMTensorToLinalgExtPass());
       return true;
     }
+
     // TODO: Retire the tm_tensor input pipeline once we are fully switched
     // to the 'torch' pipeline, which handles everything from the 'torch'
     // dialect down (vs just 'tm_tensor' which was converting a couple of
@@ -74,6 +83,7 @@ struct TorchSession
   void populateCustomInputConversionTypes(StringSet<> &typeMnemonics) override {
     typeMnemonics.insert("tm_tensor");
     typeMnemonics.insert("torch");
+    typeMnemonics.insert("onnx");
   }
 
   void populateDetectedCustomInputConversionTypes(

compiler/plugins/input/Torch/torch-mlir/CMakeLists.txt

@@ -199,6 +199,41 @@ iree_tablegen_library(
     -gen-pass-capi-impl Conversion/Passes.capi.cpp.inc
 )
 
+###############################################################################
+# TorchOnnxToTorch
+###############################################################################
+
+file(GLOB _TorchOnnxToTorchPasses_SRCS
+  "${TORCH_MLIR_ROOT_DIR}/lib/Conversion/TorchOnnxToTorch/*.cpp"
+)
+iree_cc_library(
+  NAME
+    TorchOnnxToTorchPasses
+  SRCS
+    "${TORCH_MLIR_ROOT_DIR}/lib/Conversion/TorchOnnxToTorch/Passes.cpp"
+    ${_TorchOnnxToTorchPasses_SRCS}
+  DEPS
+    ::defs
+    ::TorchConversionDialectIR
+    ::TorchDialectIR
+    ::TorchOnnxToTorchPassesGen
+    MLIRArithDialect
+    MLIRFuncDialect
+    MLIRIR
+    MLIRPass
+)
+
+iree_tablegen_library(
+  NAME
+    TorchOnnxToTorchPassesGen
+  TD_FILE
+    "${TORCH_MLIR_ROOT_DIR}/include/torch-mlir/Conversion/TorchOnnxToTorch/Passes.td"
+  OUTS
+    -gen-pass-decls Conversion/TorchOnnxToTorch/Passes.h.inc
+    -gen-pass-capi-header Conversion/TorchOnnxToTorch/Passes.capi.h.inc
+    -gen-pass-capi-impl Conversion/TorchOnnxToTorch/Passes.capi.cpp.inc
+)
+
 ###############################################################################
 # CAPI
 ###############################################################################