Why do cv::cuda::Stream need GPU memory?

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I was looking into an issue regarding cv::cuda::Stream reported here.

Attempting to solve this issue, I've found out that cv::cuda::Stream::Impl has Ptr<GpuMat::Allocator> allocator as its member and the issue was related to unallocating GPU memory allocated by this allocator object.

But why do Stream objects need a GPU memory allocator?