[RAG] Fix retrieval offset in RAG's HfIndex and better integration tests
Address @yjernite 's comment in https://github.com/huggingface/transformers/pull/7129#discussion_r488904472

Indeed the retriever was returning the indexes offset by one.

Cc @ola13 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7372?src=pr&el=h1) Report
> Merging [#7372](https://codecov.io/gh/huggingface/transformers/pull/7372?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/571c7a11c17bd00ba3e79f4d853cc51428a14e45?el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `88.88%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7372/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7372?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7372      +/-   ##
==========================================
- Coverage   77.64%   77.63%   -0.01%     
==========================================
  Files         181      181              
  Lines       35722    35728       +6     
==========================================
+ Hits        27736    27738       +2     
- Misses       7986     7990       +4     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7372?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/retrieval\_rag.py](https://codecov.io/gh/huggingface/transformers/pull/7372/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9yZXRyaWV2YWxfcmFnLnB5) | `91.01% <88.88%> (-0.27%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7372/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `85.46% <0.00%> (-0.76%)` | :arrow_down: |

------

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<|||||>@lhoestq - can you check how the `RUN_SLOW` tests would change in this case? <|||||>> @lhoestq - can you check how the `RUN_SLOW` tests would change in this case?

They change indeed. I updated the expected values.<|||||>@yjernite - could you take a final look and approve if everything seems fine to you? <|||||>Okey great - this should be the last big fix for RAG. I'll rebase this PR and merge it after 