* Stop printing out message.
* Remove R specialization.
The printed message is not really useful anyway, without a reproducible example
there's no way to fix it. But if there's a reproducible example, we can always
obtain these information by a debugger. Removing the `printf` function avoids
creating the context in kernel.
* Implement GK sketching on GPU.
* Strong tests on quantile building.
* Handle sparse dataset by binary searching the column index.
* Hypothesis test on dask.
* Move get transpose into cc.
* Clean up headers in host device vector, remove thrust dependency.
* Move span and host device vector into public.
* Install c++ headers.
* Short notes for c and c++.
Co-Authored-By: Philip Hyunsu Cho <chohyu01@cs.washington.edu>
