Solved by KFrank in post #2 
              
              
                Hi jjhh! 

The short story is that torch.minimum() and torch.maximum() are not 
differentiable at the special points where the arguments are equal. 
The function abs (x) offers a simpler example.  When x > 0, the derivative 
is +1, while when x < 0, it’s -1.  When x = 0, mathematically speaking, 
th…