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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def polish_model(model):  # type: (ModelProto) -> ModelProto
    '''
        This function combines several useful utility functions together.
    '''
    onnx.checker.check_model(model)
    onnx.helper.strip_doc_string(model)
    model = onnx.shape_inference.infer_shapes(model)
    model = onnx.optimizer.optimize(model)
    onnx.checker.check_model(model)
    return model