Fix longformer attention mask type casting when using apex
Fix for issue [#4525](https://github.com/huggingface/transformers/issues/4525). 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4574?src=pr&el=h1) Report
> Merging [#4574](https://codecov.io/gh/huggingface/transformers/pull/4574?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b86e42e0ac1b59f21f0eccf351d3346bbe3ed4eb&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4574/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4574?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4574      +/-   ##
==========================================
- Coverage   78.09%   78.08%   -0.01%     
==========================================
  Files         123      123              
  Lines       20624    20624              
==========================================
- Hits        16106    16105       -1     
- Misses       4518     4519       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4574?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4574/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `97.40% <100.00%> (ø)` | |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4574/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.44% <0.00%> (-0.42%)` | :arrow_down: |

------

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