code: def prepare_data(self):
      # download
      MNIST(str(lab.get_data_path()), train=True, download=True)
      MNIST(str(lab.get_data_path()), train=False, download=True)
  
imports: import pytorch_lightning
  import torch
  from pytorch_lightning.metrics.functional import accuracy
  from torch import nn
  from torch.nn import functional
  from torch.utils.data import DataLoader, random_split
  from torchvision import transforms
  from torchvision.datasets import MNIST
  from labml import lab, experiment
  from labml.utils.lightning import LabMLLightningLogger
label_desc: Use image node_id 7 for calling the LitMNIST obj's underlying member method code with example usage: obj.prepare_data() without return types