Set the number of times to evaluate per epoch when using Trainer
Basically, I want to see how the training is going against the evaluation dataset twice per epoch when using `Trainer`. To calculate `logging_steps` so that `evaluate()` is called a certain number of times per epoch when training, will this hold-up to single and multi-device training?

```
evals_per_epoch = 2
logging_steps = len(train_dataset) / train_batch_size / gradient_accumulation_steps // evals_per_epoch
```

I'm getting myself confused over the implementation of `global_steps` and `logging_steps` inside of `Trainer.train`. 
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