Implement Layer-wise Relevance Propagation (LRP) for prediction explanation
## 🚀 Feature

Example Code: 
https://github.com/lena-voita/the-story-of-heads/blob/master/lib/layers/attn.py#L154

## Motivation

The motivation is prediction explainability to be able to generate pictures like: 
![image](https://user-images.githubusercontent.com/1810412/72099957-b5fc3a00-32ef-11ea-8740-0d050809ea48.png)
or
![image](https://user-images.githubusercontent.com/1810412/72100012-d1674500-32ef-11ea-92ef-44917e08bf0b.png)

more motivation: http://www.heatmapping.org/slides/2019_ICCV.pdf

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>I second this! :+1: 