Layers that require cuDNN or cuBLAS capabilities have handles as member object. A useless handle is created when they are default constructed. They are going to be replaced by the actual handle later. Handle creation is costly. Hence, all the temporary handles waste resources.

Each cv::dnn::Net object now creates exactly one stream, one cuDNN handle and one cuBLAS handle.

This PR makes default constructed handle objects not create any handle. A special constructor must be used to create a handle.

Others:

• drop support for default streams (for better error detection)
• do not allow moving empty handles into other handles (for better error detection)
• fix handle leak (no bugs yet since the bugged move constructor isn't being used)
• avoid creating shared pointers for empty UniqueHandle objects

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 OpenCV (BSD) 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

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