Context that construct cnn in the auxiliary arm.

def switch_to_aux_top_layer(self):
        """Context that construct cnn in the auxiliary arm."""
        if self.aux_top_layer is None:
            raise RuntimeError("Empty auxiliary top layer in the network.")
        saved_top_layer = self.top_layer
        saved_top_size = self.top_size
        self.top_layer = self.aux_top_layer
        self.top_size = self.aux_top_size
        yield
        self.aux_top_layer = self.top_layer
        self.aux_top_size = self.top_size
        self.top_layer = saved_top_layer
        self.top_size = saved_top_size