code: def on_validation_epoch_end(self):
      if not self.validation_step_outputs:
          return
      self.log(
          "val_loss",
          torch.stack(self.validation_step_outputs).mean(),
          sync_dist=True,
      )
      self.validation_step_outputs.clear()
  
imports: import os
  import shutil
  import pytest
  import torch
  from omegaconf import OmegaConf
  from pytorch_lightning import Trainer
  from nemo.core import ModelPT
  from nemo.utils import logging
  from nemo.utils.exp_manager import CallbackParams, ExpManagerConfig, StatelessTimer, exp_manager
label_desc: Use image node_id 11 for calling the ExampleModel obj's underlying member method code with example usage: obj.on_validation_epoch_end() without return types