'Apply the backward pass transformation to the input data.
Arguments:
error (Tensor): deltas back propagated from the adjacent higher layer
Returns:
Tensor: deltas to propagate to the adjacent lower layer'

@Layer.accumulates
def bprop(self, error):

    if (self.deltas is None):
        self.deltas = error.reshape(self.y.shape)
    self.be.sum(self.deltas, axis=1, out=self.dW)
    return error
