keras/src/layers/rnn/bidirectional.py/Bidirectional/build
class Bidirectional:    def build(self, sequences_shape, initial_state_shape=None):
        if not self.forward_layer.built:
            self.forward_layer.build(sequences_shape)
        if not self.backward_layer.built:
            self.backward_layer.build(sequences_shape)
        self.built = True