Fix dynamic argmax/min when select_last_index is set (#827)

Signed-off-by: Kevin Chen <kevinch@nvidia.com>

Author: kevinch-nv

onnx2trt_utils.cpp

@@ -108,21 +108,25 @@ NodeImportResult argMinMaxHelper(IImporterContext* ctx, const ::ONNX_NAMESPACE::
     // We don't care about the TopK values, just the indices.
     nvinfer1::ITensor* indices = layer->getOutput(1);
     indices->setType(nvinfer1::DataType::kINT32);
+
+    // If selectLastIndex is true, the TopK operation was performed on reversed data on the provided axis.
+    // Convert reversed indices back to forward indices by calculating the following:
+    // indices = shape(tensor)[axis] - indices - 1
     if (selectLastIndex)
     {
-        nvinfer1::Dims dims = tensor.getDimensions();
-        int dimOnAxis = dims.d[axis];
-        nvinfer1::Dims resultShape(dims);
-        resultShape.d[axis] = 1;
-        ShapedWeights shapeWeights = ctx->createTempWeights(::ONNX_NAMESPACE::TensorProto::INT32, resultShape);
-        std::vector<int> tempData(shapeWeights.count(), dimOnAxis);
-        std::memcpy(shapeWeights.values, tempData.data(), shapeWeights.count() * sizeof(int));
+        // Use shapeTensor semantics to support dynamic shapes
+        auto const dims = shapeOf(tensor);
+        auto const indicesDims = shapeOf(*indices);
+        auto const axisTensor = shapeVector(axis);
+        auto const dimOnAxis = gather(ctx, dims, axisTensor);
+
+        // Create constant of shape indicesDims with values tensor.shape[axis]
+        auto const tensorDimOnAxis = constantOfShape(ctx, node, &dimOnAxis.tensor(ctx), &indicesDims.tensor(ctx));
 
-        ShapedWeights weightOfOnes = ctx->createTempWeights(::ONNX_NAMESPACE::TensorProto::INT32, resultShape);
-        std::vector<int> ones(shapeWeights.count(), 1);
-        std::memcpy(weightOfOnes.values, ones.data(), weightOfOnes.count() * sizeof(int));
+        // Create constant of shape indicesDims with values of 1
+        auto const ones = constantOfShape(ctx, node, &shapeVector(1).tensor(ctx), &indicesDims.tensor(ctx));
 
-        std::vector<TensorOrWeights> newInputs{shapeWeights, indices, weightOfOnes};
+        std::vector<TensorOrWeights> newInputs{tensorDimOnAxis, indices, ones};
         indices = &elementwiseHelper(ctx, node, newInputs, nvinfer1::ElementWiseOperation::kSUB).value().at(0).tensor();
     }
     if (keepdims)