title: Find input shape from onnx file
body: <p>How can I find the input size of an onnx model? I would eventually like to script it from python.</p>
  
  <p>With tensorflow I can recover the graph definition, find input candidate nodes from it and then obtain their size. Can I do something similar with ONNX (or even simpler)?</p>
  
  <p>Thank you</p>