code: def forward(
      self,
      xs_pad: torch.Tensor,
      ilens: torch.Tensor,
  ) -> Tuple[torch.Tensor, torch.Tensor, Optional[torch.Tensor]]:
      raise NotImplementedError
  
imports: from abc import ABC, abstractmethod
  from typing import Optional, Tuple
  import torch
label_desc: Use image node_id 2 for calling the AbsGenerator obj's underlying member method code with example usage: obj.forward(xs_pad, ilens) without return types