Write report and monitoring methods for the losses

[einbandi]

Users should be able to monitor the losses after training to get a better idea of weights in compound losses and/or appropriate learning rates. I still need to think about how to best handle compound losses, as they can in theory be arbitrarily nested. In the beginning it might be best to just have a flat dict for each training phase with all losses and their values throughout the epochs.

[einbandi]

Down the line it would be nice to have some more advanced reporting methods that also create visuals, but this is not important now.