Initialize the predictor module for running prediction.

def _init_predictor(self, input_shapes, type_dict=None):
        """Initialize the predictor module for running prediction."""
        shapes = {name: self.arg_params[name].shape for name in self.arg_params}
        shapes.update(dict(input_shapes))
        if self._pred_exec is not None:
            arg_shapes, _, _ = self.symbol.infer_shape(**shapes)
            assert arg_shapes is not None, "Incomplete input shapes"
            pred_shapes = [x.shape for x in self._pred_exec.arg_arrays]
            if arg_shapes == pred_shapes:
                return
        # for now only use the first device
        pred_exec = self.symbol.simple_bind(
            self.ctx[0], grad_req='null', type_dict=type_dict, **shapes)
        pred_exec.copy_params_from(self.arg_params, self.aux_params)

        _check_arguments(self.symbol)
        self._pred_exec = pred_exec