NormedByDerivative's derivatives are incorrect. #58
Description
When doing NormedByDerivative<WithGradient<..., ...>, ..., ...>, the resulting gradient is incorrect. This is tracked now in an ignored test. It may be that this simply is not differentiable, as the test is very close and the math seems correct. And yet it doesn't pass.
I don't think this is a big deal in practice, but this issues is open to track any issues you may hit with this. If you want to try you hand at the calculus, go for it! But I'm inclined to let it slide unless anyone finds a practical problem with a use case.