Feature Request: El-Attention
# 🚀 Feature request
I've looked into the paper titled "[EL-Attention: Memory Efficient Lossless Attention for Generation](https://arxiv.org/abs/2105.04779)".
It proposes a method for calculating attention that forgoes creating multi-head attention from the hidden state. This saves computational time and frees memory. 

## Motivation
El-attention seems to have no downsides, and promises significant memory and performance gains during training and inference. 

## Your contribution
The main difficulty may be in that it requires being added directly in to each model's attention mechanism code, or requires a ton of new subclasses for each part of each model. Maybe an easier solution to this would be a pipeline to use custom  attention mechanism code. 