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

@staticmethod
def _check_grad_list(grad_list):

    nvars = [len(k) for k in grad_list]
    assert (len(set(nvars)) == 1), ('Number    of    gradients    from    each    tower    is    different!    ' + str(nvars))
