code: def _to_tensor(self):
      assert "axis" in self.onnx_attr, "axis is a required attribute"
      axis = int(self.onnx_attr["axis"])
      data = [i.zvalue for i in self.model_inputs]
      return zlayers.Merge(mode="concat", concat_axis=axis)(data)
  
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
label_desc: Use image node_id 2 for calling the ConcatMapper obj's underlying member method code with example usage: obj._to_tensor() and returns: zlayers