cxx17

First of all, great library!

I am having some confusion in understanding the role of values_first2 in set_difference_by_key as mentioned here.

In general terms, the result of set difference D = A - B will not contain values from B therefore values_first2 can never be part of D. So `val

There is some inconsistency in the documentation for cub::DeviceSpmv (https://nvlabs.github.io/cub/structcub_1_1_device_spmv.html). The overview says

The SpMV computation performs the matrix-vector operation y = alpha*A*x + beta*y,

But the function signature of CsrMV has neither alpha nor beta, and the function's description says

This function performs the matrix-vector