code: def validation_step(self, *args, **kwargs):
      epoch_trainer_idx = self._get_trainer_idx_from_epoch()
      return self._trainers[epoch_trainer_idx].validation_step(
          *args, **kwargs
      )
  
imports: import bisect
  import functools
  import itertools
  from collections import OrderedDict
  from typing import Dict, List, Tuple
  import torch.nn
  from pytorch_lightning.loops.optimization.optimizer_loop import ClosureResult
  from reagent.core.utils import lazy_property
  from .reagent_lightning_module import ReAgentLightningModule
label_desc: Use image node_id 14 for calling the MultiStageTrainer obj's underlying member method code with example usage: obj.validation_step() and returns: self