I've added an extra check for the gradient ( I used to have it in my branch,
because I think at one point I got an error because of that, though I'm not
100% sure we should need to do that check).

Since the name gets into the C code, this would cause different modules
(with different C code) to have equal keys.

output is not differentiable.

Sorry guys, this copying things from one side to the other is harder then I
thought, but I think I figured out how to do it ;)

Lop is based on the grad methods that are already implemented, while Rop
requires rop methods for each op that are going to be pushed soon.