class_docstr: adapted from https://pytorch.org/tutorials/intermediate/spatial_transformer_tutorial.html
code: def forward(self, x):
      if self.is_ref:
          return self.stn_ref(x)
      return self.stn(x)
  
imports: from __future__ import annotations
  import unittest
  import numpy
  import torch
  import torch.nn
  import torch.nn.functional
  import torch.optim
  from monai.data import create_test_image_2d
  from monai.networks.layers import AffineTransform
  from monai.utils import set_determinism
  from tests.utils import DistTestCase, TimedCall
label_desc: Use image node_id 4 for calling the STNBenchmark obj's underlying member method code with example usage: obj.forward(x) and returns: self, self