This will allow to override CUDA CMake variables from first CMake call,
like cmake -DCUDA_TOOLKIT_ROOT_DIR=... -DCUDA_CUDA_LIBRARY=...

do not unset variables if CUDA_TOOLKIT_TARGET_DIR is not defined

if they are not needed

put it into core documentation

Signed-off-by: Yan Wang <yan.wang@linux.intel.com>

* use opaque algorithm interfaces
* add stream support

it caused runtime failures