Misplaced compound_l1_mssim_loss

[Daval-G]

The function named compound_l1_mssim_loss is currently defined in models/training/compile.py which is not really intuitive. I would expect it to be defined along with other loss functions at evaluate.metrics.tf_metrics.py or simply in evaluate.metrics.

[zaccharieramzi]

Good point. I would rather have a new file, losses.py where we could define this, because the compound mssim is not really a metric per se.