Thanks for your answer, i appreciate i too much but my doubt hasnt been solved yet 
After of reading the oficial Faster RCNN paper
The documents describe 3 different methods

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The method that did you explained me is one of the training method explained in the paper?
If not, why Pytorch is using this method of training?
Sorry whether i am asking something trivial but now i am in a point where i beginning to understand lot of things (and questions come)