New abstractions: Add "EdgeBasedGFlowNet" and "StateBasedGFlowNet"

[saleml]

Similar to TrajectoryBasedGFlowNet, which is the superclass of TB, SubTB, and LogZVar GFlowNets, meaning that the loss function decomposes into a sum of losses over trajectories, we might need to have two more useful abstractions, as mentioned in the Foundations paper: EdgeBasedGFlowNet and StateBasedGFlowNet