Adding RAG to text-generation pipeline
# 🚀 Feature request

Thank you for the awesome work. I am working on https://github.com/deepset-ai/haystack/issues/443 and just wanted to check whether any plan to add RAG into `text-generation` pipeline.

## Motivation

`text-generation` already have other models, hence it I would be great to have it in there. And this will help keeping our code clean by not adding classes for each type of generators.

```
model = pipeline('text-generation', model="facebook/rag-token-nq", tokenizer=None, device=-1)
# ValueError: Unrecognized configuration class <class 'transformers.configuration_rag.RagConfig'> for this kind of AutoModel: AutoModelForCausalLM.
Model type should be one of CamembertConfig, XLMRobertaConfig, RobertaConfig, BertConfig, OpenAIGPTConfig, GPT2Config, TransfoXLConfig, XLNetConfig, XLMConfig, CTRLConfig, ReformerConfig, BertGenerationConfig.
```

## Your contribution

If you guide me I am happy to help.

Hey @lalitpagaria - RAG is quite different to other generation models so we don't have it on the short-term roadmap to add it to pipelines. We are still thinking about how to integrate retrieval augmented models to the pipelines.<|||||>Thanks @patrickvonplaten 
Yeah I totally agree with you.
Please let me know whether I close this issue or keep it open for future reference.<|||||>Leave it open - I'll put it under projects so that I don't forget it :-) 
<|||||>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.
<|||||>@patrickvonplaten Great work! I noticed that `transformers` included the implementation for `DPR`. But for `RAG`, I only find a [demo](https://huggingface.co/rag/). Is there a source code for `RAG`? Or do you know where is Facebook's source code for `RAG`? <|||||>transformers does include RAG.
You can even find the documentation here: https://huggingface.co/transformers/model_doc/rag.html