report/plots: combine train/val/test metrics plots

[dberenbaum]

Some of the callbacks write out train/val/test metrics. Can we combine these onto one plot instead of plotting them all separately?

[daavoo]

This doesn't feel like a DVCLive responsibility to me. Our UIs should have an easy way of building top-level plots from existing individual plots

[dberenbaum]

That might be true, but if we already are opinionated about how to organize and name those metrics in the callback, why not be opinionated about how to plot them?