This function converts a PyTorch Geometric `Data` object representing a directed graph into its line digraph representation.

to_line_digraph.py

    def to_line_digraph(self, data: Data) -> Data:
        """
        TODO can we make this more efficient by removing the for-loop?
        """
        assert data.edge_index is not None
        assert data.is_directed()
        edge_index, edge_attr = data.edge_index, data.edge_attr
        N, E = data.num_nodes, data.num_edges

        edge_index, edge_attr = coalesce(edge_index, edge_attr, num_nodes=data.num_nodes)
        row, col = edge_index

        new_edge_index = []
        for i in range(E):
            new_col_i = torch.nonzero(row == col[i])
            new_row_i = i * torch.ones_like(new_col_i)
            new_edge_index.append(torch.cat([new_row_i, new_col_i], dim=1))
        new_edge_index = torch.cat(new_edge_index, dim=0).t()

        data.edge_index = new_edge_index
        data.x = edge_attr
        data.num_nodes = E
        data.edge_attr = None
        return data