Fast influence functions

[epwalsh]

Add an implementation of "FastIF" (https://arxiv.org/abs/2012.15781) to the interpret.influence_functions module.

[dirkgr]

This is not what @leo-liuzy was working on?

[epwalsh]

Leo's main project was the "simple" implementation of Influence Functions. We discussed FastIF as a potential follow-up project, but didn't have time to get to it. I think it would be a valuable addition to the library though, so that's why I created this feature request.

[github-actions]

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