Add support for multiple metrics for validation

[george-gca]

Is your feature request related to a problem? Please describe.
My model uses multiple metrics for validation, and it would be useful if I could visualize each one of them.

Describe the solution you'd like
When sending info to server, it would be nice if a dict of metrics (composed by its names and current value) could be provided.

Describe alternatives you've considered
Maybe it would be nice to split the sendLoss function into different functions, like sendLoss, sendMetrics.

Additional context
For cases when working with Super Resolution, one should compare images using regular metrics (e.g. PSNR) and metrics that consider human perception (e.g. SSIM), for example.