[Matrix]: implement get_underlying_mats in mem_matrix_store.
[Matrix]: implement get_underlying_mats in mem_matrix_store.

this is a temporary solution.

flashxio/FlashX,flashxio/FlashX,icoming/FlashX,icoming/FlashGraph,icoming/FlashX,flashxio/FlashX,icoming/FlashX,flashxio/FlashX,icoming/FlashX,icoming/FlashGraph,flashxio/FlashX,icoming/FlashGraph,flashxio/FlashX,icoming/FlashGraph,icoming/FlashX,icoming/FlashGraph