code: def forward(self, x, cat_vec=None):
      _, hidden_state = self.backbone(x)
      pos, h = hidden_state[-1]
      h, _ = self.ptb(self.fc(h), pos)
  
      for i in range(self.n_blocks):
          h = self.transition_ups[i](
              pos, h, hidden_state[-i - 2][0], hidden_state[-i - 2][1]
          )
          pos = hidden_state[-i - 2][0]
          h, _ = self.transformers[i](h, pos)
      return self.out(torch.cat([h, cat_vec], dim=-1))
  
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 PointTransformerSeg obj's underlying member method code with example usage: obj.forward(x, cat_vec) and returns: self