Args:
            all_packed: K lists of packed gradients.

def unpack_all(self, all_packed, devices):
        """
        Args:
            all_packed: K lists of packed gradients.
        """
        all_grads = []  # #GPU x #Var
        for dev, packed_grads_single_device in zip(devices, all_packed):
            with tf.device(dev):
                all_grads.append(self.unpack(packed_grads_single_device))
        return all_grads