[Bart] add imports to examples
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3160?src=pr&el=h1) Report
> Merging [#3160](https://codecov.io/gh/huggingface/transformers/pull/3160?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/6ffe03a0a1d472a4e5941793fd361d2b82c8be3f?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3160/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3160?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3160      +/-   ##
==========================================
+ Coverage   78.11%   78.12%   +0.01%     
==========================================
  Files          98       98              
  Lines       16651    16651              
==========================================
+ Hits        13007    13009       +2     
+ Misses       3644     3642       -2
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3160?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/3160/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `93.41% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3160/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `94.72% <0%> (+0.31%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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