docstr: :return: list of OnnxInput
code: def _extract_model_inputs(self):
      """
      :return: list of OnnxInput
      """
  
      def gen_input(input):
          if isinstance(input.zvalue, np.ndarray):
              return self._to_zoo_input(input, is_constant=True)
          else:
              return self._to_zoo_input(input)
  
      return [gen_input(i) for i in self._input_list]
  
file_path: analytics-zoo/pyzoo/zoo/pipeline/api/onnx/mapper/matmul.py
imports: from zoo.pipeline.api.onnx.mapper.operator_mapper import OperatorMapper
  from zoo.pipeline.api.onnx.onnx_helper import OnnxHelper
  import zoo.pipeline.api.keras.layers
  import zoo.pipeline.api.autograd
  import numpy
label_desc: Use image node_id 2 for calling the MatMulMapper obj's underlying member method code with example usage: obj._extract_model_inputs() and returns: unknown, self, self