text: Title: Add TopK node to a pretrained Brevitas model
  Body: We are working with FINN-ONNX, and we want the pretrained models from Brevitas that classify the MNIST images to output the index (class) instead of a probabilities tensor of dim 1x10.To our knowledge, the node responsible for this is the TopK. 
  
  Where do we have to add this layer, and what function can we add so the 'export_qonnx' would understand it as a TopK node?
  
  The desired block is in the following image:
  ![Screenshot from 2024-02-09 16-56-07](https://github.com/onnx/onnx/assets/92207421/2b5cd758-a044-4b99-928a-5f8f51c22a6f)
   