[onnx] Fix onnx.Shape lowering with scalar input (#3716)

Address nod-ai/SHARK-ModelDev#826

Author: jinchen62

lib/Conversion/TorchOnnxToTorch/DefaultDomainQtoZ.cpp

@@ -1662,29 +1662,29 @@ void mlir::torch::onnx_c::populateDefaultDomainQtoZ(
         auto shapeType = Torch::ValueTensorType::get(
             binder.op->getContext(), SmallVector<int64_t>{inputRank},
             resultType.getOptionalDtype());
-
         Value shape = rewriter.create<Torch::Aten_ShapeAsTensorOp>(
             binder.getLoc(), shapeType, operand);
 
+        if (inputRank == 0) {
+          rewriter.replaceOpWithNewOp<Torch::TensorStaticInfoCastOp>(
+              binder.op, resultType, shape);
+          return success();
+        }
+
         if (start == 0 && end == -1) {
           rewriter.replaceOp(binder.op, shape);
           return success();
         }
 
         Value sv = rewriter.create<Torch::ConstantIntOp>(
             binder.getLoc(), rewriter.getI64IntegerAttr(start));
-
         Value ev = rewriter.create<Torch::ConstantIntOp>(
             binder.getLoc(), rewriter.getI64IntegerAttr(end));
-
         Value step = rewriter.create<Torch::ConstantIntOp>(binder.getLoc(), 1);
-
         Value dim = rewriter.create<Torch::ConstantIntOp>(binder.getLoc(), 0);
 
-        shape = rewriter.create<Torch::AtenSliceTensorOp>(
-            binder.getLoc(), resultType, shape, dim, sv, ev, step);
-
-        rewriter.replaceOp(binder.op, shape);
+        rewriter.replaceOpWithNewOp<Torch::AtenSliceTensorOp>(
+            binder.op, resultType, shape, dim, sv, ev, step);
         return success();
       });
 

test/Conversion/TorchOnnxToTorch/simple_ops_q_to_z.mlir

@@ -2833,6 +2833,15 @@ func.func @test_shape_start_1_end_negative_1(%arg0: !torch.vtensor<[3,4,5],f32>)
   return %0 : !torch.vtensor<[1],si64>
 }
 
+// -----
+
+// CHECK-LABEL: func.func @test_shape_scalar
+func.func @test_shape_scalar(%arg0: !torch.vtensor<[],si64> ) -> !torch.vtensor<[?],si64>  attributes {torch.onnx_meta.ir_version = 8 : si64, torch.onnx_meta.opset_version = 17 : si64, torch.onnx_meta.producer_name = "pytorch", torch.onnx_meta.producer_version = "2.1.0"} {
+  // CHECK: %[[SHAPE:.+]] = torch.aten._shape_as_tensor %arg0 : !torch.vtensor<[],si64> -> !torch.vtensor<[0],si64>
+  // CHECK: %[[CAST:.+]] = torch.tensor_static_info_cast %[[SHAPE]] : !torch.vtensor<[0],si64> to !torch.vtensor<[?],si64>
+  %0 = torch.operator "onnx.Shape"(%arg0) : (!torch.vtensor<[],si64>) -> !torch.vtensor<[?],si64>
+  return %0: !torch.vtensor<[?],si64>
+}
 
 // -----