code: def forward(self, logits, y):
      num_classes = logits.shape[1]
      logits = (
          logits.permute(0, 2, 1).contiguous().view(-1, num_classes)
      )
      loss = self.loss(logits, y)
      return loss
  
imports: import numpy
  import torch
  from helper import index_points, square_distance, TransitionDown, TransitionUp
  from torch import nn
label_desc: Use image node_id 2 for calling the PartSegLoss obj's underlying member method code with example usage: obj.forward(logits, y) and returns: loss