code: def forward(self, x):
      n, _, _, _ = x.size()
      result = x.view(n, -1)
  
      return result
  
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import os
  import tempfile
  import logging
  import unittest
  import numpy
  import torch
  import torch.nn
  import torch.optim
  from art.estimators.classification.pytorch import PyTorchClassifier
  from art.estimators.classification.detector_classifier import DetectorClassifier
  from tests.utils import TestBase, get_image_classifier_pt
label_desc: Use image node_id 1 for calling the Flatten obj's underlying member method code with example usage: obj.forward(x) and returns: result