CeresMinimizer from bqskitrs makes a call to num_residuals on DifferentiableResidualsFunction objects. The string num_residuals is not present anywhere in the Python bqskit repo, making it unclear that this is a requirement for DifferentiableCostFunction.

Also HilbertSchmidtResiduals does not implement num_residuals, which is problematic for what I am trying to do (I'm writing a residuals function that modifies another residuals function).