[trainer] fractional epoch 
Running 
```
export BS=20; rm -r output_dir; CUDA_VISIBLE_DEVICES=0,1 PYTHONPATH=../../src USE_TF=0   ./finetune_trainer.py --model_name_or_path t5-small --output_dir output_dir --adam_eps 1e-06 --data_dir wmt_en_ro --do_eval --do_predict --do_train --evaluation_strategy=steps --freeze_embeds --label_smoothing 0.1 --learning_rate 3e-5 --logging_first_step --logging_steps 1000 --max_source_length 128 --max_target_length 128 --num_train_epochs 1 --overwrite_output_dir --per_device_eval_batch_size $BS --per_device_train_batch_size $BS --predict_with_generate --eval_steps 25000  --sortish_sampler --task translation_en_to_ro --test_max_target_length 128 --val_max_target_length 128 --warmup_steps 5 --n_train 100 --n_val 100 --n_test 100  --fp16 --save_steps 1
```
on master, gives:
```
{'loss': 14.846837043762207, 'learning_rate': 6e-06, 'epoch': 0.3333333333333333} 
```
epoch can't be fractional.

The fraction (and float) `'epoch': 0.3333333333333333` comes from here:

https://github.com/huggingface/transformers/blob/1c19b423bf274a465f95725a79819bf82f71329e/src/transformers/trainer.py#L899

@sgugger - is this by design 

or should it be `ceil`: `'epoch': 1`
```
                    self.state.epoch = math.ceil( epoch + (step + 1) / steps_in_epoch)
```
or `floor`:  `'epoch': 0`
```
                    self.state.epoch = math.floor( epoch + (step + 1) / steps_in_epoch) 
```

`'epoch': 0.3333333333333333` is telling me it's somewhere in the first epoch but isn't done yet?

Perhaps it's just fine, it's just very odd to see epoch not being an int.

Thanks.
<|||||>This is not a bug, it's completely normal. See the [dcoumentation](https://huggingface.co/transformers/main_classes/callback.html#trainerstate) of `TrainerState.epoch`.<|||||>Ah, OK. Some rounding then perhaps - `0.3333333333333333` is just too loud. 2 decimals?<|||||>Sure, there is no formatting at all for those results, but we can add some.<|||||>Anything else to format while I'm at it? loss I guess - 4 decimals, right?

`{'loss': 14.846837043762207, 'learning_rate': 6e-06, 'epoch': 0.3333333333333333} `<|||||>I don't see why not.