docstr: Serialize ONNX model to JSON message 
docstr_tok: ["Serialize","ONNX","model","to","JSON","message"]
code: def __init__(self, model):
      """Serialize ONNX model to JSON message"""
      # import onnx.shape_inference
      # model = onnx.shape_inference.infer_shapes(model)
      self.value = model
      self.metadata = _Metadata()
      self.graph = _Graph(model.graph, self.metadata)
  
imports: import collections
  import enum
  import json
  import os
label_desc: Use image node_id 1 to create a new _Model object  with example: obj = _Model(model)