Create a variable graph given  network by name

        Returns: NnpNetwork

def get_network(self, name, batch_size=None, callback=None):
        '''Create a variable graph given  network by name

        Returns: NnpNetwork

        '''
        network_proto = nnabla_pb2.Network()
        network_proto.CopyFrom(self.network_dict[name])
        return NnpNetwork(network_proto, self._params, batch_size, callback=callback)