Warn if any element of the gradient is not updated by eval or if it's zeroed

[Mikolaj]

If a gradient is not updated by eval, this is either an experiment ("what happens if I remove this expression") or a typo causing unused parameter. We can detect this for vector or matrix gradients, but not for scalar gradients.

Gradients that get zeroed are more common, but may be a bad sign and in case of scalars may be an unused parameter.