LineDiGraph transform: added option to specify which node features wi…

…ll become edge features in the line digraph

torch_geometric/transforms/line_digraph.py

@@ -21,12 +21,26 @@ class LineDiGraph(BaseTransform):
 
     Line-digraph node indices are equal to indices in the original graph's
     coalesced :obj:`edge_index`.
+
+    Args:
+        node_to_edge_features (str, optional): If set to :obj:`'none'`, the
+            node attributes of the original graph will not be put into the line
+            digraph. Otherwise, for each line digraph edge
+            :obj:`e = ((u, v), (v, x))`, :obj:`'inner'` will use node
+            attributes of :obj:`v` as edge attributes, :obj:`'outer'` will use
+            node attributes of :obj:`u` and :obj:`x` as edge attributes, and
+            :obj:`'all'` will use node attributes of :obj:`u`, :obj:`v` and
+            :obj:`x` as edge attributes.
     """
+    def __init__(self, node_to_edge_features: str = 'none'):
+        assert node_to_edge_features in ['none', 'inner', 'outer', 'all']
+        self.node_to_edge_features = node_to_edge_features
+
     def forward(self, data: Data) -> Data:
         assert data.edge_index is not None
         assert data.is_directed()
         edge_index, edge_attr = data.edge_index, data.edge_attr
-        E = data.num_edges
+        new_num_nodes = data.num_edges
 
         edge_index, edge_attr = coalesce(edge_index, edge_attr, data.num_nodes)
         row, col = edge_index
@@ -35,8 +49,30 @@ def forward(self, data: Data) -> Data:
         mask = row.unsqueeze(0) == col.unsqueeze(1)  # (num_edges, num_edges)
         new_edge_index = torch.nonzero(mask).T
 
+        new_num_edges = new_edge_index.size(1)
+
+        # Obtain new edge attributes
+        if data.x is None or self.node_to_edge_features == 'none':
+            new_edge_attr = None
+        else:
+            node_features = data.x
+            edge_src = row[new_edge_index[0]]
+            edge_mid = col[new_edge_index[0]]
+            edge_dst = col[new_edge_index[1]]
+
+            if self.node_to_edge_features == "inner":
+                new_edge_attr = node_features[edge_mid]
+            elif self.node_to_edge_features == "outer":
+                new_edge_attr = torch.cat([node_features[edge_src],
+                                           node_features[edge_dst]], dim=-1)
+            else:  # self.node_to_edge_features == "all"
+                new_edge_attr = torch.cat([node_features[edge_src],
+                                           node_features[edge_mid],
+                                           node_features[edge_dst]], dim=-1)
+
         data.edge_index = new_edge_index
         data.x = edge_attr
-        data.num_nodes = E
-        data.edge_attr = None
+        data.num_nodes = new_num_nodes
+        data.num_edges = new_num_edges
+        data.edge_attr = new_edge_attr
         return data