code: def __call__(self, x):
      return F.sum((x - self.w) ** 2)
  
imports: import pickle
  import unittest
  import numpy
  import six
  import chainer
  from chainer import functions
  from chainer import optimizers
  from chainer import testing
label_desc: Use image node_id 2 for calling the SimpleChain obj's underlying member method code with example usage: obj.__call__(x) and returns: F