[trainer] round numbers in trainer state

[stas00]

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

[LysandreJik]

LGTM!