small bug in RAG model
## Environment info
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     Don't forget to fill out the missing fields in that output! -->

- `transformers` version: 4.5.1
- Platform: Linux-5.4.0-66-generic-x86_64-with-glibc2.27
- Python version: 3.8.3
- PyTorch version (GPU?): 1.8.1+cu111 (True)

### Who can help
@ola13 

Models:

- rag

## Information

Model I am using (Bert, XLNet ...): RAG

The problem arises when using:
* [ ] the official example scripts: seems to be a bug in https://github.com/huggingface/transformers/blob/master/src/transformers/models/rag/modeling_rag.py#L306
To pass my own question encoder the argument needs to be `question_encoder_model`, but the suffix ("model") gets trimmed because the len is taken according to `question_question_encoder_`.

```python
kwargs_question_encoder = {
            argument[len("question_question_encoder_") :]: value
            for argument, value in kwargs.items()
            if argument.startswith("question_encoder_")
        }
```

## To reproduce

Steps to reproduce the behavior:

```python
question_encoder = AutoModel.from_pretrained("any model")
rag_model = model_class.from_pretrained_question_encoder_generator(
        question_encoder_model=question_encoder, generator_pretrained_model_name_or_path=generator_name_or_path, config=rag_config
    )
```

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## Expected behavior

be able to pass a question encoder **model** and not just a config to the rag model.

This indeed looks like a typo! Thanks for your issue :-)

The kwargs should not be trimmed by `"question_question_encoder"`, but by `"question_encoder"`.

Would you like to open a PR to fix it? Otherwise I can do it as well :-)<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Unping