[fix] add bart to LM_MAPPING
This fixes 2/5 failing slow tests in #6094 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6099?src=pr&el=h1) Report
> Merging [#6099](https://codecov.io/gh/huggingface/transformers/pull/6099?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/06834bc33255f5fb8fabb72c9ff114764b3c7ce5&el=desc) will **increase** coverage by `0.16%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6099/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6099?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6099      +/-   ##
==========================================
+ Coverage   77.77%   77.93%   +0.16%     
==========================================
  Files         146      146              
  Lines       26325    26325              
==========================================
+ Hits        20474    20517      +43     
+ Misses       5851     5808      -43     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6099?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/6099/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `78.48% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/6099/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9ncHQyLnB5) | `65.42% <0.00%> (-29.91%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6099/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `85.71% <0.00%> (-0.51%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6099/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `98.79% <0.00%> (+33.89%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6099?src=pr&el=footer). Last update [06834bc...1c7f573](https://codecov.io/gh/huggingface/transformers/pull/6099?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
