Many warnings with -Wconversion

[nvibd]

We're seeing a lot of warnings on Linux, host compiler GCC 9.3.0 with -Wconversion.

For a basic example, compile:

#include <thrust/device_vector.h>
int main() {
  thrust::device_vector<int> a;
  return 0;
}

With: nvcc main.cu -Xcompiler=-Wconversion.

This will result in around 1000 lines of output warnings.
Tested with the Thrust versions in CUDA 11.0, 11.3, and the latest commit on main on this repository (776bbc4d).

[alliepiper]

We should add this flag to Thrust and CUB's CI build system for all targets.

[jrhemstad]

This should be resolved by #531