Merge pull request #24461 from fengyuentau:tracker_vit_backend_target

Video tracking (dnn): set backend and target for TrackerVit #24461

Resolves #24460

### Pull Request Readiness Checklist

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

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

modules/video/src/tracking/tracker_vit.cpp

@@ -45,6 +45,9 @@ public:
     {
         net = dnn::readNet(params.net);
         CV_Assert(!net.empty());
+
+        net.setPreferableBackend(params.backend);
+        net.setPreferableTarget(params.target);
     }
 
     void init(InputArray image, const Rect& boundingBox) CV_OVERRIDE;