You need to change the get method of your dataset (x is already array type).
torch::data::Example<> get(size_t index) override {
   std::vector<float> x = std::get<0>(csv_[index]);
   float y = std::get<1>(csv_[index]);

   torch::Tensor x_input = torch::tensor( x ); // - removed braces here
   torch::Tensor y_label = torch::tensor({ y });

   return { x_input, y_label };
};