Error raised by "tmp_eval_loss += tmp_eval_loss.item()" when using multi-gpu
fixed the bug raised by "tmp_eval_loss += tmp_eval_loss.item()" when parallelly using multi-gpu.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1608?src=pr&el=h1) Report
> Merging [#1608](https://codecov.io/gh/huggingface/transformers/pull/1608?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ef1b8b2ae5ad1057154a126879f7eb8de685f862?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1608/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1608?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master   #1608      +/-   ##
=========================================
+ Coverage   86.17%   86.2%   +0.02%     
=========================================
  Files          91      91              
  Lines       13595   13595              
=========================================
+ Hits        11715   11719       +4     
+ Misses       1880    1876       -4
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1608?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/1608/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2ZpbGVfdXRpbHMucHk=) | `76.37% <0%> (+2.19%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1608?src=pr&el=footer). Last update [ef1b8b2...bd847ce](https://codecov.io/gh/huggingface/transformers/pull/1608?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Thanks @focox!