Fix a bug in eval_batch_retrieval of eval_rag.py
# What does this PR do?

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Following the instructions in [RAG example](https://github.com/huggingface/transformers/tree/master/examples/research_projects/rag#retrieval-evaluation), I was trying to evaluate retrieval against DPR evaluation data.

`pipenv run python eval_rag.py     --model_name_or_path facebook/rag-sequence-nq     --model_type rag_sequence     --evaluation_set output/biencoder-nq-dev.questions     --gold_data_path output/biencoder-nq-dev.pages     --predictions_path output/retrieval_preds.tsv      --eval_mode retrieval     --k 1`

With the above command, I faced the following error and confirmed that `question_enc_outputs` is a tuple whose length is 1.

```
...
loading weights file https://huggingface.co/facebook/rag-sequence-nq/resolve/main/pytorch_model.bin from cache at /home/ubuntu/.cache/huggingface/transformers/9456ce4ba210322153f704e0f26c6228bd6c0caad60fe1b3bdca001558adbeca.ee816b8e716f9741a2ac602bb9c6f4d84eff545b0b00a6c5353241bea6dec221
All model checkpoint weights were used when initializing RagSequenceForGeneration.

All the weights of RagSequenceForGeneration were initialized from the model checkpoint at facebook/rag-sequence-nq.
If your task is similar to the task the model of the checkpoint was trained on, you can already use RagSequenceForGeneration for predictions without further training.
initializing retrieval
Loading index from https://storage.googleapis.com/huggingface-nlp/datasets/wiki_dpr/
loading file https://storage.googleapis.com/huggingface-nlp/datasets/wiki_dpr/hf_bert_base.hnswSQ8_correct_phi_128.c_index.index.dpr from cache at /home/ubuntu/.cache/huggingface/transformers/a481b3aaed56325cb8901610e03e76f93b47f4284a1392d85e2ba5ce5d40d174.a382b038f1ea97c4fbad3098cd4a881a7cd4c5f73902c093e0c560511655cc0b
loading file https://storage.googleapis.com/huggingface-nlp/datasets/wiki_dpr/hf_bert_base.hnswSQ8_correct_phi_128.c_index.index_meta.dpr from cache at /home/ubuntu/.cache/huggingface/transformers/bb9560964463bc761c682818cbdb4e1662e91d25a9407afb102970f00445678c.f8cbe3240b82ffaad54506b5c13c63d26ff873d5cfabbc30eef9ad668264bab4
7it [00:00, 212.77it/s]
Traceback (most recent call last):
  File "eval_rag.py", line 315, in <module>
    main(args)
  File "eval_rag.py", line 301, in main
    answers = evaluate_batch_fn(args, model, questions)
  File "eval_rag.py", line 99, in evaluate_batch_retrieval
    question_enc_pool_output = question_enc_outputs.pooler_output
AttributeError: 'tuple' object has no attribute 'pooler_output'
```

With this simple change (`question_enc_outputs.pooler_output` -> `question_enc_outputs[0]`), I got to run the evaluation code and confirmed 
`INFO:__main__:Precision@1:  70.74`

## Environments
- Ubuntu 18.04 LTS
- Python 3.7.7
- transformers 4.0.1
- torch: 1.7.1

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


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@ola13 (confirmed by `git blame`) @patrickvonplaten @lhoestq 
@lhoestq - feel free to merge if you're ok with the PR