Bart: fix layerdrop and caching shapes for generation
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2969?src=pr&el=h1) Report
> Merging [#2969](https://codecov.io/gh/huggingface/transformers/pull/2969?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c36416e53c29da8b6193f4a36d7b024c5f513495?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2969/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2969?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2969      +/-   ##
==========================================
- Coverage   77.14%   77.14%   -0.01%     
==========================================
  Files          98       98              
  Lines       16006    16003       -3     
==========================================
- Hits        12348    12345       -3     
  Misses       3658     3658
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2969?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/2969/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `84.58% <100%> (-0.11%)` | :arrow_down: |

------

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