Loss metric
Overall, the PR looks good to merge. Please fix the tests and final review from @MaximumEntropy please. 
Add a PTL metric for averaging loss across processes in ddp mode.

Motivation.
If ddp is used, the validation loss for only one gpu is logged. This behavior impairs "Reduce on plateau" and "Early stopping" scheduling strategies. If many GPUs are used and the validation dataset is small, logged validation loss can differ from real validation loss a lot.