Clipping in gradient_wrt_input

[jankrepl]

Currently, we implement one "Explainable" algorithm in deepdow.explain.gradient_wrt_input. The problem is that we do not restrict the values the input can have. One solution would be to implement some projection/clipping logic that takes place after each optimizer step and thus forces the values to be in a given range.

See https://arxiv.org/pdf/1702.04782.pdf