BartForSequenceClassification: fix num_labels, add test
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3110?src=pr&el=h1) Report
> Merging [#3110](https://codecov.io/gh/huggingface/transformers/pull/3110?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5c5af879b6d45c879c987154f66d4ea978925fb2?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3110/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3110?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3110      +/-   ##
==========================================
+ Coverage   77.83%   77.84%   +<.01%     
==========================================
  Files          98       98              
  Lines       16422    16422              
==========================================
+ Hits        12782    12783       +1     
+ Misses       3640     3639       -1
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3110?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/3110/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `93.38% <100%> (+0.33%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3110/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `94.45% <0%> (-0.16%)` | :arrow_down: |

------

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