[Matrix]: fix a minor bug in groupby.
[Matrix]: fix a minor bug in groupby.

I need to optimize for the case that the label vector is stored on
disks.

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