Bart can make decoder_input_ids from labels
mimics a nice t5 feature
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6758?src=pr&el=h1) Report
> Merging [#6758](https://codecov.io/gh/huggingface/transformers/pull/6758?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a75c64d80c76c3dc71f735d9197a4a601847e0cd?el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6758/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6758?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6758      +/-   ##
==========================================
- Coverage   78.96%   78.96%   -0.01%     
==========================================
  Files         157      157              
  Lines       28486    28488       +2     
==========================================
+ Hits        22495    22496       +1     
- Misses       5991     5992       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6758?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6758/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `95.57% <100.00%> (+0.01%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6758/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.41% <0.00%> (-0.26%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6758?src=pr&el=footer). Last update [a75c64d...06e7cc5](https://codecov.io/gh/huggingface/transformers/pull/6758?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
