“…The scalable solution of highdimensional outer-loop problems often requires derivative information, since this information detects map sensitivities that can make problems effectively low-dimensional. This property has been observed and used in many outer-loop problems such as model reduction for sampling and deep learning [4,5,13,34,36], optimization under uncertainty [3,15,17], Bayesian inverse problems [6,9,10,11,12,14,16,18,21,23,26,44], and Bayesian optimal experimental design [2,20,39,40,41].…”