Sparse updates

THIS PR need to be rebased on master after gh-1489 is merged. The first commit of the current PR will disapear. as it is already in gh-1489.

NEWS.txt
- safer case to int32 in sparse op CSM. (Frederic B.)
- BUGFIX: disable the optimization that introduce the CSMGradC op as it don't work correctly with unsorted indices. (Frederic B.)