fix parse content in CreatePreLoadReaders (#20258)
fix parse content in CreatePreLoadReaders (#20258)

Fix parse content in CreatePreLoadReaders. Before this fix, if you use dataset.set_parse_content and dataset.preload, parse content didn't work.
PaddlePaddle/Paddle,chengduoZH/Paddle,luotao1/Paddle,chengduoZH/Paddle,luotao1/Paddle,chengduoZH/Paddle,chengduoZH/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,chengduoZH/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,luotao1/Paddle,PaddlePaddle/Paddle