Guarantee DT + T is compatible

[thomasahle]

Right now, when taking the derivative with respect to a tensor with edges i, j, k, ... we name them, by default, i', j', k'.
However, a common pattern is to add a gradient, like d/dx y^T x to a tensor, x. So we should ensure they have the same shape/edge names when possible.
We should also mention this convention in the book