Update DNN Optical Flow sample with RAFT model #24424
Description
Describe the feature and motivation
Originally, sample https://github.com/opencv/opencv/blob/4.x/samples/dnn/optical_flow.py created for FlowNet v2. Recently, @Aser-Abdelfatah and @fengyuentau enabled support of RAFT model. Sample should work with both models.
Additional context
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