Add epoch number to all log messages during training

[tilmankamp]

Log messages of new best validating checkpoints and plateau encounters don't carry epoch numbers. This makes it harder than necessary to assess training progress using grep.

[DanBmh]

I like this, always had to search it myself in the logs... But I found out today that you also can run cat your_logfile | grep -B1 Saved to show the line before the selected line too.