Combined with `onnx.load(<filename>)` to run onnxinfo

[ExplorerRay]

Present implementation requires another read_onnx(<filename>) to get the ONNX file, but python and ONNX users usually import onnx, I think it is better not to read the same file two times.

[ExplorerRay]

Finding that the type of model in onnx.load and my own _onnxinfo.read_onnx is different.

_onnxinfo.read_onnx: <class '_onnxinfo.ModelProto'>
onnx.load: <class 'onnx.onnx_ml_pb2.ModelProto'>