flow and node_dim

[mdanb]

What exactly do the parameters flow and node_dim for MessagePassing determine?

[rusty1s]

• flow dicates the flow of message passing, which corresponds to the way nodes send messages to their neighbors. In case flow="source_to_target", nodes will sent messages along their edges (a source node sends a message and a target node receives a message). flow="target_to_source" reverses this computation flow. This might be useful if you want to distinguish messages in an directed graph (where you also want to send messages in the reversed graph).

• node_dim denotes the dimension in which you want to aggregate node features. This is set to node_dim=-2 by default, but you may want to customize it in case your node features do not have shape [num_nodes, num_features]. For example, you may think of temporal node features of shape [num_nodes, seq_length, num_features], where you want to aggregate node features in the first dimension. Here, you need to set node_dim to 0 or -3.