[Longformer] fix longformer global attention output
This PR fixes the attention probs that are outputted when longformer uses global attention and sets `output_attention=True`.

Thanks a million to @k141303 for very clean issue + perfect proposed solution in https://github.com/huggingface/transformers/issues/5646 .
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5659?src=pr&el=h1) Report
> Merging [#5659](https://codecov.io/gh/huggingface/transformers/pull/5659?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0267668c3d648c6e41afda97f5df8671ee880ac3&el=desc) will **decrease** coverage by `0.13%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5659/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5659?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5659      +/-   ##
==========================================
- Coverage   77.01%   76.87%   -0.14%     
==========================================
  Files         128      145      +17     
  Lines       21615    25369    +3754     
==========================================
+ Hits        16646    19502    +2856     
- Misses       4969     5867     +898     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5659?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/5659/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.24% <ø> (+0.11%)` | :arrow_up: |
| [src/transformers/benchmark/benchmark.py](https://codecov.io/gh/huggingface/transformers/pull/5659/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrLnB5) | `74.01% <ø> (+5.16%)` | :arrow_up: |
| [src/transformers/benchmark/benchmark\_args.py](https://codecov.io/gh/huggingface/transformers/pull/5659/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrX2FyZ3MucHk=) | `86.04% <ø> (+0.68%)` | :arrow_up: |
| [src/transformers/benchmark/benchmark\_args\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/5659/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrX2FyZ3NfdGYucHk=) | `87.50% <ø> (ø)` | |
| [src/transformers/benchmark/benchmark\_args\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5659/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrX2FyZ3NfdXRpbHMucHk=) | `89.13% <ø> (-7.75%)` | :arrow_down: |
| [src/transformers/benchmark/benchmark\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/5659/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrX3RmLnB5) | `61.53% <ø> (ø)` | |
| [src/transformers/benchmark/benchmark\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5659/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmsvYmVuY2htYXJrX3V0aWxzLnB5) | `69.84% <ø> (-3.26%)` | :arrow_down: |
| [src/transformers/configuration\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/5659/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2FsYmVydC5weQ==) | `100.00% <ø> (ø)` | |
| [src/transformers/configuration\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/5659/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2F1dG8ucHk=) | `93.18% <ø> (+0.32%)` | :arrow_up: |
| [src/transformers/configuration\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/5659/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2JhcnQucHk=) | `93.75% <ø> (+0.41%)` | :arrow_up: |
| ... and [118 more](https://codecov.io/gh/huggingface/transformers/pull/5659/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5659?src=pr&el=footer). Last update [bfacb2e...ee88c2f](https://codecov.io/gh/huggingface/transformers/pull/5659?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Global and Local attention probs have now always the same output shape. This is both more consistent in terms of the output signature for the user and solves the multi-gpu issue.<|||||>Pinging @thomwolf @sshleifer @LysandreJik @sgugger for notification -> more details can be found in issue: https://github.com/huggingface/transformers/issues/5646. 