code: def _check_variables_are_known(self, variables):
      for v in variables:
          if self._var_key(v) not in self._trainable_variables_indices:
              raise ValueError(
                  f"Unknown variable: {v}. This optimizer can only "
                  "be called for the variables it was originally built with. "
                  "When working with a new set of variables, you should "
                  "recreate a new optimizer instance."
              )
  
imports: import re
  import warnings
  import numpy
  from keras import backend
  from keras import initializers
  from keras import ops
  from keras.optimizers.schedules import learning_rate_schedule
  from keras.saving import serialization_lib
  from keras.utils import tracking
  from keras.utils.naming import auto_name
label_desc: Use image node_id 9 for calling the BaseOptimizer obj's underlying member method code with example usage: obj._check_variables_are_known(variables) without return types