keras/src/layers/layer.py/Layer/_track_variable
class Layer:    def _track_variable(self, variable):
        if variable.trainable:
            self._tracker.add_to_store("trainable_variables", variable)
        else:
            self._tracker.add_to_store("non_trainable_variables", variable)
        if not self.trainable:
            variable.trainable = False
        self._post_track_variable(variable)