Adds to every token node the list of spans (span node IDs) that it
        belongs to.

        TokenNode.spans - a list of `int` ids of `SpanNode`s

def _add_span_node_ids_to_token_nodes(self):
        """
        Adds to every token node the list of spans (span node IDs) that it
        belongs to.

        TokenNode.spans - a list of `int` ids of `SpanNode`s
        """
        span_dict = defaultdict(list)
        for span_edge in self._spanning_relation_ids:
            token_node_id = self.edges[span_edge].target
            span_node_id = self.edges[span_edge].source
            span_dict[token_node_id].append(span_node_id)

        for token_node_id in span_dict:
            self.nodes[token_node_id].spans = span_dict[token_node_id]