sign typo in crossentropy computation

Author: Frederik Rahbaek Warburg

stochman/nnj.py

@@ -1194,4 +1194,4 @@ def forward(self, x: Tensor) -> Tensor:
 
     def _jacobian(self, x: Tensor, val: Tensor) -> Tensor:
         jac = 0.5 / val
-        return jac
+        return jac