Knowledge Retrieval missing from BlenderBot Implementation

# 🚀 Feature request

The original Blenderbot [paper](https://arxiv.org/pdf/2004.13637.pdf) considered three transformer based models (Retrieval, Generator and RetNRef), however from what I can see only the generator model is implemented within this repository: [transformers/src/transformers/models/blenderbot/modeling_blenderbot.py](https://github.com/huggingface/transformers/blame/master/src/transformers/models/blenderbot/modeling_blenderbot.py).

## Motivation

As part of my academic work I am generating topic bound conversations and wish to compare Blenderbot as well as make modifications to its knowledge retrieval component. It would be useful if someone could point me towards this (if it is already implemented) or inform me if this feature is planned to be added in the future.


## Contribution

Prior to making a contribution I want to confirm that feature is not implemented and there is no intention of implementing this in the near future.

Thanks,
Alex

@patrickvonplaten @patil-suraj