backwardvoid backward(Variable!(T, tensorDims, HostStorage) gx, Variable!(T, tensorDims, HostStorage) x, Variable!(T, tensorDims, HostStorage) gy, Variable!(T, tensorDims, HostStorage) y, int[poolDims] windowA, int[poolDims] padA, int[poolDims] strideA)
reference implementaion of pooling function