Fix (non-slow) tests on GPU (torch)
Fixes ~29 failing tests

Also cf previous commit from december: https://github.com/huggingface/transformers/pull/2055/commits/61978c1dd3f340a545e74537c3dae41a4514e867

(not sure why T5 and the common methods were not failing back then)
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3024?src=pr&el=h1) Report
> Merging [#3024](https://codecov.io/gh/huggingface/transformers/pull/3024?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/bb7c46852051f7d031dd4be0240c9c9db82f6ed9?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3024/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3024?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3024   +/-   ##
=======================================
  Coverage   77.26%   77.26%           
=======================================
  Files          98       98           
  Lines       16047    16047           
=======================================
  Hits        12399    12399           
  Misses       3648     3648
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3024?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/3024/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `84.58% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3024?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3024?src=pr&el=footer). Last update [bb7c468...9c14bc7](https://codecov.io/gh/huggingface/transformers/pull/3024?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
