'Backward propagate pooling layer.
Arguments:
layer (PoolLayer): The pool layer object. Different backends have
different pool layers.
I (Tensor): Input tensor.
E (Tensor): Error tensor.
grad_I (Tensor): Gradient tensor (delta)'

@abc.abstractmethod
def bprop_pool(self, layer, I, E, grad_I):

    raise NotImplementedError()
