Knowledge Retrieval missing from BlenderBot Implementation

# 🚀 Feature request <!-- A clear and concise description of the feature proposal. Please provide a link to the paper and code in case they exist. --> 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 <!-- Please outline the motivation for the proposal. Is your feature request related to a problem? e.g., I'm always frustrated when [...]. If this is related to another GitHub issue, please link here too. --> 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 <!-- Is there any way that you could help, e.g. by submitting a PR? Make sure to read the CONTRIBUTING.MD readme: https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md --> 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