code: def _get_sync_model_vars_op(self):
          
          ops = []
          for (shadow_v, local_v) in self._shadow_model_vars:
              ops.append(shadow_v.assign(local_v.read_value()))
          assert len(ops)
          return tf.group(*ops, name='sync_{}_model_variables_to_ps'.format(len(ops)))
docs: Get the op to sync local model_variables to PS.