code: def run_step(self):
          
          if not hasattr(self, 'train_op'):
              raise NotImplementedError(
                  "Please either set `Trainer.train_op` or provide an implementation "
                  "of Trainer.run_step()!")
          self.hooked_sess.run(self.train_op)
docs: Defines what to do in one iteration. The default is:
          ``self.hooked_sess.run(self.train_op)``.
  
          The behavior of each iteration can be changed by either setting ``trainer.train_op``,
          or overriding this method.