Conversion to nn.Module & Softmax added in forward

[gsarti]

Description

• The AttributionModel and child classes are now torch.nn.Module. Consequently, score_func becomes the regular forward method, simplifying the code in the attribution methods and enabling us to eliminate the HookableModelWrapper class.

• Following some examples, the output of the forward function of HuggingfaceModel is passed through a torch.softmax function. The effect of this operation is an increased sparsity in the saliency patterns, although most salient trends are preserved.