keras/src/optimizers/adadelta.py/Adadelta/build
class Adadelta:    def build(self, var_list):
        if self.built:
            return
        super().build(var_list)
        self._accumulated_grads = []
        self._accumulated_delta_vars = []
        for var in var_list:
            self._accumulated_grads.append(
                self.add_variable_from_reference(var, "accumulated_grad")
            )
            self._accumulated_delta_vars.append(
                self.add_variable_from_reference(var, "accumulated_delta_var")
            )