[Rag] Fix wrong usage of `num_beams` and `bos_token_id` in Rag Sequence generation
Author: patrickvonplaten
Pull request: yes

Small changes => big impact. Hopefully e2e results are better now @ola13 

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7386?src=pr&el=h1) Report
> Merging [#7386](https://codecov.io/gh/huggingface/transformers/pull/7386?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/8d3bb781ee2643ad1076f4cbcc6f417245671e94?el=desc) will **increase** coverage by `2.51%`.
> The diff coverage is `0.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7386/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7386?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7386      +/-   ##
==========================================
+ Coverage   76.61%   79.12%   +2.51%     
==========================================
  Files         181      181              
  Lines       35759    35760       +1     
==========================================
+ Hits        27395    28295     +900     
+ Misses       8364     7465     -899     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7386?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_rag.py](https://codecov.io/gh/huggingface/transformers/pull/7386/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yYWcucHk=) | `25.32% <0.00%> (-51.66%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_lxmert.py](https://codecov.io/gh/huggingface/transformers/pull/7386/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9seG1lcnQucHk=) | `22.14% <0.00%> (-72.41%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/7386/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `58.52% <0.00%> (-34.74%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/7386/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `65.26% <0.00%> (-33.64%)` | :arrow_down: |
| [src/transformers/modeling\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/7386/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tYXJpYW4ucHk=) | `60.00% <0.00%> (-30.00%)` | :arrow_down: |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/7386/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `74.14% <0.00%> (-18.70%)` | :arrow_down: |
| [src/transformers/activations.py](https://codecov.io/gh/huggingface/transformers/pull/7386/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9hY3RpdmF0aW9ucy5weQ==) | `79.16% <0.00%> (-4.17%)` | :arrow_down: |
| [src/transformers/configuration\_rag.py](https://codecov.io/gh/huggingface/transformers/pull/7386/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3JhZy5weQ==) | `97.77% <0.00%> (-2.23%)` | :arrow_down: |
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/7386/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `82.62% <0.00%> (-1.41%)` | :arrow_down: |
| ... and [15 more](https://codecov.io/gh/huggingface/transformers/pull/7386/diff?src=pr&el=tree-more) | |

------

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