[Matrix]: fix a compilation error.
[Matrix]: fix a compilation error.

default_vertex_index was renamed as undirected_vertex_index.

flashxio/FlashX,zheng-da/FlashX,icoming/FlashX,icoming/FlashGraph,flashxio/FlashX,flashxio/FlashX,icoming/FlashX,icoming/FlashGraph,zheng-da/FlashX,flashxio/FlashX,icoming/FlashGraph,zheng-da/FlashX,icoming/FlashGraph,icoming/FlashGraph,flashxio/FlashX,zheng-da/FlashX,icoming/FlashX,icoming/FlashX,zheng-da/FlashX,icoming/FlashX,flashxio/FlashX