code: def call_func(self, backend_config, x1, x2, x3):
      if self.function_node:
          return self.call_func_function_node(
              backend_config, x1, x2, x3
          )
      else:
          return self.call_func_function(backend_config, x1, x2, x3)
  
imports: import unittest
  import numpy
  import chainer
  from chainer import testing
  import chainer.testing.backend
  import chainerx
label_desc: Use image node_id 3 for calling the TestFunctionBackprop obj's underlying member method code with example usage: obj.call_func(backend_config, x1, x2, x3) and returns: self, self