title: How to convert channel last keras model into channel first ONNX model
body: <p>I want to convert keras .h5 model into ONNX model. But my model is trained channel last and I want it channel first.
  I found that there is an argument in <code>keras2onnx.convert_keras</code> called <code>channel_first_inputs</code> but couldn't find any example on how to use it on their <a href="https://github.com/onnx/keras-onnx/blob/master/README.md" rel="nofollow noreferrer">official site</a>.</p>
  <p>I am doing this step as a part of the process of converting my keras model into <em>.engine</em> model. Is there any other way to do so without the need to onnx intermediate step?.</p>
  <hr />
  <p>Searching for the parameter name inside the keras2onnx code, I found the following usage</p>
  <ul>
  <li>In <a href="https://github.com/onnx/keras-onnx/blob/239908eefbba216c3f138364b14af31a8fd2b18b/applications/yolov3/yolov3.py" rel="nofollow noreferrer">Here</a>: <strong>channel_first_inputs=['input_1']</strong></li>
  <li>In <a href="https://github.com/onnx/keras-onnx/blob/17559f987ecce7ec40ab8a36a9596eb950f9b332/tests/test_layers.py" rel="nofollow noreferrer">Here</a>: <strong>channel_first_inputs=[model.input_names[0]]</strong></li>
  </ul>