Add a graph to demonstrate GPU/XPU scaling

User story: _"As a reader of the report, I'd like to see a graph of the epoch timings against GPU/XPU count, so I can get a better idea about how scaling affects training time."_

As we collect epoch timings for distributed computation, it might help make the numbers clearer if we could plot them on a graph.

Effective epoch time against number of processes (equivalent to XPU or GPU number) maybe?