code: def on_validation_start(
      self, trainer: Trainer, pl_module: LightningModule
  ):
      logger.info("Evaluation time. Running on full validation set...")
      self.snapshot_timer.reset()
      self.val_combined_report = None
      pl_module.val_meter.reset()
  
imports: import logging
  from typing import Any, Dict, List
  import torch
  from mmf.common.meter import Meter
  from mmf.common.registry import registry
  from mmf.common.report import Report
  from mmf.common.sample import SampleList
  from mmf.utils.logger import calculate_time_left, summarize_report
  from mmf.utils.timer import Timer
  from pytorch_lightning import LightningModule, Trainer
  from pytorch_lightning.callbacks.base import Callback
label_desc: Use image node_id 4 for calling the LightningLoopCallback obj's underlying member method code with example usage: obj.on_validation_start(trainer, pl_module) without return types