def split_backward(layers):  
backward = []<EOL>forward = [sink_return(op.begin_update, backward.append) for op in layers]<EOL>return forward, backward<EOL>
Separate a sequence of layers' `begin_update` methods into two lists of
    functions: one that computes the forward values, and the other that completes
    the backward pass. The backward sequence is only populated after the forward
    functions have been applied.