code: def step(self, epoch=None):
      if epoch is None:
          epoch = self.last_epoch + 1
  
      self.last_epoch = epoch
      self.model.apply(self.setter(self.lmbd(epoch)))
  
imports: import numpy
  import matplotlib.pyplot
  from mpl_toolkits.mplot3d import Axes3D
  import random
  import torch
  import torch.nn
  import torch.nn.functional
  import os
  from collections import abc
label_desc: Use image node_id 2 for calling the BNMomentumScheduler obj's underlying member method code with example usage: obj.step(epoch) without return types