Args:
            grad_list: list of list of tuples, shape is Ngpu x Nvar x 2

def _check_grad_list(grad_list):
        """
        Args:
            grad_list: list of list of tuples, shape is Ngpu x Nvar x 2
        """
        nvars = [len(k) for k in grad_list]

        def basename(x):
            return re.sub('tower[0-9]+/', '', x.op.name)

        if len(set(nvars)) != 1:
            names_per_gpu = [set([basename(k[1]) for k in grad_and_vars]) for grad_and_vars in grad_list]
            inters = copy.copy(names_per_gpu[0])
            for s in names_per_gpu:
                inters &= s
            for s in names_per_gpu:
                s -= inters
            logger.error("Unique trainable variables on towers: " + pprint.pformat(names_per_gpu))
            raise ValueError("Number of gradients from each tower is different! " + str(nvars))