I think you just raise a mistake from my side ! Indeed, I want to do a binary segmentation but I think wrongly design my model by outputting 2 values instead of 1 and then taking the max. I am kind of new to the field so an error happened quickly … 
So what I should do is have only one output from my model (a softmax probability or a logit ?) and then giving to the nn.BCELoss() ?
Thank you for your big help (as always)