Integrated Gradients serialization with layers that are not accessible via model.layers

[jklaise]

Because of https://github.com/SeldonIO/alibi/pull/398 it is not clear how to handle serialization of IG explainers specifying custom layers not accessible via model.layers.

[jklaise]

This is fairly important to resolve as we don't serialize the original model as part of serializing the explainer, so by extension we can't serialize the layer object, but if there is no way to refer to it otherwise than just by having a reference to it then we're stuck...