[Matrix]: fix a bug in getting portions from groupby matrix.
[Matrix]: fix a bug in getting portions from groupby matrix.

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