Complete Grad-CAM, Transformers + SHAP (GSoC)

[teabolt]

Hi,

I would like to work on ELI5 as a GSOC student again. I would like to merge the Grad-CAM PR's #325 and #327, work on extra features like adding tensorflow support, and look into explaining Transformer networks. If there is time I would also like to look into adding SHAP #299.

@ivanprado

Thanks,
Tomas