[trainer] round numbers in trainer state
This PR rounds very long fractions in trainer state e.g., 

```
{'loss': 14.846837043762207, 'learning_rate': 6e-06, 'epoch': 0.3333333333333333} 
```
to:
* epoch 2 decimals
* loss 4 decimals

resulting in:

```
{'loss': 14.8468, 'learning_rate': 6e-06, 'epoch': 0.33}
```` 

If you want any other small tweaks for me to add please let me know.

Fixes: #9475 

@sgugger 