Support to record validation loss

[MeowZheng]

Describe the feature

Motivation

Now, val_loop cannot return the loss, and it is inconvenient when users would like to record and monitor the validation loss.

Related resources

https://github.com/open-mmlab/mmsegmentation/pull/1494

[ZwwWayne]

I remember the conclusion is to implement a new ValLoop. We may provide a design in documentation and support it in the future.

[jeannotes]

any progress? @ZwwWayne

[pbamotra]

any progress?