code: def _prepare_dec_target(
      self,
      dec_source: List[int],
      clean_input_tokens: List[int],
      vocab: Vocabulary,
  ) -> List[int]:
      dec_target = [
          vocab.get_pad_index()
          if dec_source_token != vocab.get_mask_index()
          else dec_real_target_token
          for (dec_source_token, dec_real_target_token) in zip(
              dec_source, clean_input_tokens
          )
      ]
  
      return dec_target
  
imports: from typing import Dict, List, Optional, Set
  import numpy
  from pytext.common.constants import SpecialTokens, Token
  from pytext.config.component import Component, ComponentType
  from pytext.config.pytext_config import ConfigBase
  from pytext.data.data_structures.annotation import Annotation, Intent, Root, Slot
  from pytext.data.utils import VocabBuilder, Vocabulary
label_desc: Use image node_id 5 for calling the MaskingFunction obj's underlying member method code with example usage: obj._prepare_dec_target(dec_source, clean_input_tokens, vocab) and returns: dec_target