tangent * tangent and dot(tangent, tangent) can return tangents.

[lukas-weber]

I noticed that there are methods of * and dot for the AbstractZero types that work like

Base.:*(::NoTangent, ::NoTangent) = NoTangent()
LinearAlgebra.dot(::NoTangent, ::ZeroTangent) = ZeroTangent()

However, if both arguments are tangents,

1. * should not be defined.
2. dot should return a real number, not another tangent.

In my understanding, code that relies on such nonlinear tangent to tangent operations cannot be correct, so it may be worthwhile to remove such methods.