code: def cost(self, chain: Chain):
      if chain is not None:
          return chain.ftime[self.index]
      else:
          return 1
  
imports: import math
  from abc import ABC
  from typing import List
  from torch.utils._pytree import tree_map
label_desc: Use image node_id 2 for calling the Forward obj's underlying member method code with example usage: obj.cost(chain) and returns: chain, int