Bart: Use bool attention_mask for encoder
Wasn't breaking because bool(-1e4) is True, but clearer this way.
I believe the `bool` operator was introduced in PyTorch 1.2.0, won't this break compatibility with PyTorch 1.0.0?<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3054?src=pr&el=h1) Report
> Merging [#3054](https://codecov.io/gh/huggingface/transformers/pull/3054?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/8bcb37bfb80d77e06001f989ad982c9961a69c31?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3054/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3054?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3054      +/-   ##
==========================================
- Coverage    77.2%   77.18%   -0.02%     
==========================================
  Files          98       98              
  Lines       16063    16063              
==========================================
- Hits        12401    12399       -2     
- Misses       3662     3664       +2
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3054?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/3054/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `84.58% <100%> (ø)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3054/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `92.22% <0%> (-0.33%)` | :arrow_down: |

------

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