Adding a translation end-to-end example.
This is an attempt to get translation example but end-to-end not just finetuning (so we actually run the tokenizer's training for instance). Is this something worth anything ? 

Also, I felt a bit confused that summarizing and translation were in `seq2seq`, it might be more readable to split this folder into `summarization` and `translation`. No ?

- Using `nlp` for the dataset (using nl-en because I was using it for a personal projet, could be changed).
- The script creates `whole_data.txt` and `tokenizer.json` and `lightning_logs` which I feel should be at least in the gitignore
 even better would be contained somewhere (is there a proper place?)
- The script focused on being a single source (Only one commands runs the whole pipeline).
- It *should* be easy to swap a dataset for another or an architecture for another, or a tokenizer for another.
- Currently lacks actualy inference mode with beam search.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6288?src=pr&el=h1) Report
> Merging [#6288](https://codecov.io/gh/huggingface/transformers/pull/6288?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d5bc32ce92ace9aaec7752e0b89d51ba18903a1b&el=desc) will **decrease** coverage by `0.29%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6288/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6288?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6288      +/-   ##
==========================================
- Coverage   79.56%   79.26%   -0.30%     
==========================================
  Files         147      147              
  Lines       27125    27125              
==========================================
- Hits        21581    21500      -81     
- Misses       5544     5625      +81     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6288?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6288/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `64.47% <0.00%> (-32.95%)` | :arrow_down: |
| [src/transformers/tokenization\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6288/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcm9iZXJ0YS5weQ==) | `76.71% <0.00%> (-21.92%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/6288/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `86.43% <0.00%> (-7.42%)` | :arrow_down: |
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/6288/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `38.73% <0.00%> (-3.76%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_fast.py](https://codecov.io/gh/huggingface/transformers/pull/6288/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfZmFzdC5weQ==) | `92.14% <0.00%> (-2.15%)` | :arrow_down: |
| [src/transformers/tokenization\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6288/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fb3BlbmFpLnB5) | `82.57% <0.00%> (-1.52%)` | :arrow_down: |
| [src/transformers/tokenization\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/6288/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmVydC5weQ==) | `91.07% <0.00%> (-0.45%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6288/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `90.00% <0.00%> (-0.41%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6288/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <0.00%> (+5.76%)` | :arrow_up: |
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6288/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `95.77% <0.00%> (+35.21%)` | :arrow_up: |
| ... and [1 more](https://codecov.io/gh/huggingface/transformers/pull/6288/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6288?src=pr&el=footer). Last update [d5bc32c...c70d376](https://codecov.io/gh/huggingface/transformers/pull/6288?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>There seems to be a flaky test for run_examples. (I had a few errors with pip install step. Invalid hash... quite worrying)