Let $f(X)=\lVert UXV\rVert_*$ with $U^TU=I_m$ and $V^TV=I_n$. What is the subgradient $\partial f(X)$?
I don't know whether the answer is simply generated by the subgradient of nuclear norm, i.e., $$\partial f(X) = U^T\partial \lVert UXV^T\rVert_* V ?$$