[s2s] rougeLSum expects \n between sentences
<!-- This line specifies which issue to close after the pull request is merged. -->
Fixes #6808 
Continues #7356 from @swethmandava
Coauthor: @swethmandava

+ `add_newline_sep` kwarg controls whether to add newlines between sentences
+ test coverage
+ can pass bootstrap=False to see raw scores, make scoring deterministic.
+ Verified metrics improvement for bart on CNN/Dailymail, no change for XSUM


# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7410?src=pr&el=h1) Report
> Merging [#7410](https://codecov.io/gh/huggingface/transformers/pull/7410?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/eab5f59682cf197cd5fd19d499b3670dbef67000?el=desc) will **decrease** coverage by `0.87%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7410/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7410?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7410      +/-   ##
==========================================
- Coverage   77.77%   76.89%   -0.88%     
==========================================
  Files         181      181              
  Lines       35781    35781              
==========================================
- Hits        27828    27514     -314     
- Misses       7953     8267     +314     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7410?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/7410/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3JlZm9ybWVyLnB5) | `21.62% <0.00%> (-78.38%)` | :arrow_down: |
| [src/transformers/modeling\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/7410/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yZWZvcm1lci5weQ==) | `16.71% <0.00%> (-77.89%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/7410/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `18.73% <0.00%> (-74.53%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_lxmert.py](https://codecov.io/gh/huggingface/transformers/pull/7410/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9seG1lcnQucHk=) | `22.14% <0.00%> (-72.41%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/7410/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `21.53% <0.00%> (-68.15%)` | :arrow_down: |
| [src/transformers/tokenization\_funnel.py](https://codecov.io/gh/huggingface/transformers/pull/7410/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fZnVubmVsLnB5) | `62.79% <0.00%> (-34.89%)` | :arrow_down: |
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/7410/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `87.04% <0.00%> (+1.03%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/7410/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `82.25% <0.00%> (+1.29%)` | :arrow_up: |
| [src/transformers/configuration\_rag.py](https://codecov.io/gh/huggingface/transformers/pull/7410/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3JhZy5weQ==) | `100.00% <0.00%> (+2.22%)` | :arrow_up: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/7410/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `97.58% <0.00%> (+2.41%)` | :arrow_up: |
| ... and [8 more](https://codecov.io/gh/huggingface/transformers/pull/7410/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7410?src=pr&el=footer). Last update [eab5f59...ee83da0](https://codecov.io/gh/huggingface/transformers/pull/7410?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
