Bug in save() method of ONNXWrapper class

[shiqiangw]

Some part of the code in the save() method of ONNXWrapper class assumes a Keras model, but the load() method that uses onnx.load() returns a ModelProto model.

For ModelProto model, the following line doesn't work because ModelProto does not have layers attribute, for instance:

trustyai-ood/ood_enabler/model_wrapper/onnx.py

Line 102 in b4de64c

input_dims = self.model.layers[0].get_config()['batch_input_shape']

Some code in the save() method needs to be updated and onnx.save() should be used to save the OOD-enabled model. Parameters in the config file need to be read in a way using ModelProto methods.