

TITLE
can't set evaluation_strategy to "epoch"

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2

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## Environment info

- `transformers` version: 3.3.1

### Who can help
@sgugger 

## Information
Seems like we can't use the evaluation_strategy=epoch option. The default value of evaluate_during_training is boolean (False). In transformers/training_args.py, __post_init__ function, there is a check - `if self.evaluate_during_training is not None:` (line 326). If I'm not mistaken, it will always result in True. I think that if you change the line into `if self.evaluate_during_training:` the problem will be solved.

Thank you so much for your awesome work :)  