docstr: Our dataloaders produce a micro-batch and then we fetch
  a number of microbatches depending on the global batch size and model parallel size
  from the dataloader to produce a list of microbatches.
  The list of microbatches is then piped through the pipeline using megatron-core fwd/bwd functions.        
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code: def validation_step(self, dataloader_iter, batch_idx):
      """
      Our dataloaders produce a micro-batch and then we fetch
      a number of microbatches depending on the global batch size and model parallel size
      from the dataloader to produce a list of microbatches.
      The list of microbatches is then piped through the pipeline using megatron-core fwd/bwd functions.
      """
  
      loss, val_loss_dict = self.fwd_bwd_step(
          dataloader_iter, batch_idx, True
      )
  
      self.log_dict(
          val_loss_dict,
          prog_bar=False,
          logger=True,
          on_step=False,
          on_epoch=True,
          batch_size=1,
      )
      return loss
  
imports: import itertools
  from datetime import datetime
  from functools import partial
  from typing import Any
  import torch
  from omegaconf import DictConfig, open_dict
  from pytorch_lightning import Trainer
  from nemo.collections.multimodal.data.imagen.imagen_dataset import build_train_valid_datasets
  from nemo.collections.multimodal.models.text_to_image.imagen.precond import ContinousDDPMPrecond, EDMPrecond
  from nemo.collections.multimodal.modules.imagen.diffusionmodules.nets import EfficientUNetModel, UNetModel
  from nemo.collections.multimodal.modules.imagen.encoder.t5encoder import T5Encoder
  from nemo.collections.multimodal.modules.imagen.sampler.sampler import DDPMSampler, EDMSampler
  from nemo.collections.multimodal.parts.imagen.utils import random_dropout
  from nemo.collections.nlp.models.language_modeling.megatron_base_model import MegatronBaseModel
  from nemo.collections.nlp.modules.common.megatron.module import Float16Module
  from nemo.collections.nlp.parts.utils_funcs import get_last_rank
  from nemo.core.classes.common import Serialization
  from nemo.utils import logging
label_desc: Use image node_id 15 for calling the MegatronImagen obj's underlying member method code with example usage: obj.validation_step(dataloader_iter, batch_idx) and returns: loss