Creating a DGL graph with `create_dgl_graph` function

[pgniewko]

I've been looking into the code, and this line seems a bit puzzling (numerical experiments don't show any difference in the model's performance w/ and w/o this line). What is so special about the 'effect/phenotype' and why does it need to be set to 0? Isn't it replaced by a positive number anyways in the succeeding for loop? Are there any edge cases where even though the 'effect/phenotype' is not in the df, we still want to keep this node type in the graph?