Logging losses on the validation / test split

## Problem

I think right now logging losses (PPO or otherwise) as defined under `core/algorithms/onpolicy_sync/losses/` is only supported on the train split. If I understand correctly, right now on the train split, it's possible to log losses (and other quantities defined in the loss definitions -- policy entropy, etc.) as well as metrics (and rewards) defined in the task definitions (success, SPL, etc. under `plugins/robothor_plugin/robothor_tasks.py` for instance). However, on val, only logging the latter is supported. While logging loss values (and other quantities defined in loss definitions) on val may not be super-useful in case of PPO, but may be worthwile for debugging experiments that have additional custom defined losses (action-prediction, etc.).

## Desired solution

To be able to observe loss (and other quantities defined under loss definitions) on the val split in the tensorboard logs.