[trainer] add Trainer methods for metrics logging and saving 
This PR introduces:
* [x] `trainer.log_metrics` - to perform consistent formatting for logged metrics
* [x] `trainer.save_metrics` - to save the metrics

This removes a lot of pointless noise from the example scripts and makes them much easier to read and understand. It doesn't take away from a user understanding the example, since these helper methods are just removing formatting and file saving. 

If accepted it should be easy to replicate to other example scripts so that they all produce a consistent output and are all easier to read.

@sgugger 
@sgugger, are you ok if we merge this and I will ask Second Good Issue to help with this - I'm not sure I will have time to do this and test all the scripts in the coming days, and since you guys discuss changing this script again, we should probably merge this first.<|||||>I'm fine with that!<|||||>Started an issue here: https://github.com/huggingface/transformers/issues/10337 - This is an easy task so I think First Good Issue might work. Let me know if I should bump it to Second.
