Deprecate .from_inner2 from StructuredMatrix interface #37
Description
We originally incorporated this function to support updates based K.T @ (a @ a.T) @ K and K.T @ (g @ g.T) @ K, e.g. in the private ASDL implementation.
But now we are using an update that computes (K.T @ a) (K.T @ a).T and (K.T @ g) @ (K.T @ g).T instead via StructuredMatrix.from_inner. Hence, we can remove from_inner2 and the associated tests.