ilkarman

 update_mini_batch <- function(mini_batch, lr, C, sizes, num_layers, biases, weights)
 {
   nmb <- length(mini_batch)
   listw <- sizes[1:length(sizes)-1] 
   listb <-  sizes[-1]  

   # Initialise updates with zero vectors (for EACH mini-batch)
   nabla_b <- lapply(seq_along(listb), function(idx){
     r <- listb[[idx]]
     matrix(0, nrow=r, ncol=1)