Merge with extra: opencv/opencv_extra#943

The square layer is used to square each element. We noiced that we can implement square layer by "Eltwise" by connecting the input twice to it.

Pull Request Readiness Checklist

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• I agree to contribute to the project under Apache 2 License.
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  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake