• Each spline coupling layer (represented in blue in Figure ) used a specific non-random conditioning protocol (defined in SI Figure ). The number of trainable parameters was adjusted by varying the number of coupling layers to minimise the variance of FE estimates obtained using the BAR V estimator (defined in section II D) evaluated on the models during training. Models were trained for as long as necessary to observe a convergent behaviour in the running average of these estimates, as defined in section II E.