Dataset Format for training RAG on custom Dataset
### System Info

```shell
- `transformers` version: 4.19.2
- Platform: Linux-4.14.219-164.354.amzn2.x86_64-x86_64-with-glibc2.2.5
- Python version: 3.7.9
- Huggingface_hub version: 0.4.0
- PyTorch version (GPU?): 1.10.1+cu102 (True)
- Tensorflow version (GPU?): 2.3.0 (False)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?: no
```


### Who can help?

_No response_

### Information

- [X] The official example scripts
- [X] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [X] My own task or dataset (give details below)

### Reproduction

Steps to reproduce error -  No format mentioned for data preparation for training RAG on custom dataset.
[link to script](https://github.com/huggingface/transformers/blob/main/examples/research_projects/rag/finetune_rag.py)
[broken link to format in which data needs to be prepared](https://github.com/huggingface/transformers/tree/main/examples/research_projects/rag#:~:text=Our%20finetuning%20logic%20is%20based%20on%20scripts%20from%20examples/seq2seq.%20We%20accept%20training%20data%20in%20the%20same%20format%20as%20specified%20there%20%2D%20we%20expect%20a%20directory%20consisting%20of%206%20text%20files%3A)

### Expected behavior

```shell
https://github.com/huggingface/transformers/blob/main/examples/research_projects/rag/finetune_rag.py
This is a script to fine tune rag on a custom dataset, however the link mentioned on github (format in which data needs to be prepared if training on custom dataset) is broken. Please let me know in what format do I have to prepare my training data if I want to train RAG on a custom dataset

https://github.com/huggingface/transformers/tree/main/examples/research_projects/rag#:~:text=Our%20finetuning%20logic%20is%20based%20on%20scripts%20from%20examples/seq2seq.%20We%20accept%20training%20data%20in%20the%20same%20format%20as%20specified%20there%20%2D%20we%20expect%20a%20directory%20consisting%20of%206%20text%20files%3A
```

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