Remember that the answers are given by voluntary users in their sparetime. Then the term "as soon as possible" is not appropriate. Your question is not more urgent than the others.
You did not explain, what you call a "group". It would help to solve your problem, if you mention, what your goal is. A small example with a [6 x 3] matrix and 2 "groups" might be useful.
in this way I have Y 200*10*512 with 524288 elements. I need 200 different group with 10 of my orginal matrix. and then I can access each group of 10 for further implemetation. This one is kind of complicated
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