Fixing mode in evaluate during training
This fixs the error of not passing mode while using the option of evaluate_during_training option.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1709?src=pr&el=h1) Report
> Merging [#1709](https://codecov.io/gh/huggingface/transformers/pull/1709?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/8a62835577a2a93642546858b21372e43c1a1ff8?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1709/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1709?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1709   +/-   ##
=======================================
  Coverage   85.14%   85.14%           
=======================================
  Files          94       94           
  Lines       13920    13920           
=======================================
  Hits        11852    11852           
  Misses       2068     2068
```



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<|||||>Ok thanks!