code: def training_step(self, batch, batch_index):
      (x0, x1) = batch[0]
      z0 = self.forward(x0)
      z1 = self.forward(x1)
      loss = self.criterion(z0, z1)
      return loss
  
file_path: lightly/examples/pytorch_lightning/barlowtwins.py
imports: import pytorch_lightning
  import torch
  import torchvision
  from torch import nn
  from lightly.loss import BarlowTwinsLoss
  from lightly.models.modules import BarlowTwinsProjectionHead
  from lightly.transforms.byol_transform import BYOLTransform, BYOLView1Transform, BYOLView2Transform
label_desc: Use image node_id 3 for calling the BarlowTwins obj's underlying member method code with example usage: obj.training_step(batch, batch_index) and returns: loss