Target branching over RF and RGCN models

[jds485]

To reduce code length, it would be nice to implement targets branching for RF and RGCN model targets. Probably best to implement for the plots in 4_predict_plots.R instead of the model training targets in 4_predict.R because individual models have different specifications, whereas I think all models have the same plot function args.

Idea from @lekoenig here:

I wonder if, in a new target, we could add elements to those lists that contain the model_name and out_dir and then map over each list to populate the arguments in plot_vip.