remove default stream sync in cuda TVL1 optical flow

[noahstier]

This is an optimization in the cuda TVL1 optical flow implementation.

It moves a summation reduction off of the default cuda stream, preventing a device sync. It also eliminates a slow cudaHostAlloc and cudaHostFree.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

force_builders=Custom
build_worker:Custom=linux-4,linux-6
build_image:Custom=ubuntu-cuda:18.04

[noahstier]

This gives me much better GPU utilization for this function. I can share nvprof data if it would be helpful.

tasks:

• proper branch: In version 3.4 this function is in opencv, not opencv_contrib. Should I also make this change in 3.4 in opencv?

• bug report: I didn't make one--should I?. I am attaching a screenshot from nvprof where you can see the cudaDeviceSynchronize and the extra pinned memory alloc / free.

[alalek]

Thank you for contribution 👍

3.4 branch is in the maintenance mode.
If you need to change this branch too, then you can push changes to the main repository.