code: def on_validation_epoch_start(
      self, trainer: Trainer, pl_module: LightningModule
  ):
      self.val_start_time = time.time()
  
file_path: NeMo/examples/multimodal/x_to_nerf/benchmark_callback.py
imports: import time
  from typing import Optional
  from pytorch_lightning import Callback, LightningModule, Trainer
  from nemo.utils import logging
label_desc: Use image node_id 7 for calling the BenchmarkCallback obj's underlying member method code with example usage: obj.on_validation_epoch_start(trainer, pl_module) without return types