Move ML module to opencv_contrib

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Original issue
Describe the feature and motivation

After ~25 years of developing OpenCV and ~20 years of opencv_ml module history we found that it’s time to move ML outside of the main repository. Here are the major reasons:

  • ML module is not used in other parts of OpenCV, it’s not used in any samples or tutorials, except for samples or tutorials for ML itself. Very few, if any, people from OpenCV community actually use it.
  • Traditional ML has been completely replaced with Deep Learning in Computer Vision.
  • Traditional ML, including decision trees, boosting (especially gradient boosting), support vector machines etc. is still actively used by scientific community, but there are high-quality packages that implement state-of-art approaches, most notable scikit-learn. We don’t see any strong reason and don’t have any resources to compete with such packages.

So, we are going to move ML module to opencv_contrib together with tests, samples, docs/tutorials.