Solved by KFrank in post #4 
              
              
                Hello Abdeljalil! 

It’s probably not the issue, but please also try SGD with no 
momentum (momentum = 0.0). 

Some comments based on your edited post: 
First, get rid of the sigmoid() at the end of your model.  Thus: 
        output = self.fc3(x)
        return output

And use BCEWithLogitsLoss (ra…