Refine flowers dataset

[wanghaoshuang]

在paddle.data.flowers中，将数据以batch为单位打包成了独立的文件存放在文件系统，这种操作在多卡训练情况下容易出现问题，先在需要对其修改。
删除对batch_images_from_tar的调用：https://github.com/PaddlePaddle/Paddle/blob/develop/python/paddle/dataset/flowers.py#L126

并参考paddle.vision.datasets.flowers和mnist重构paddle.data.flowers：
paddle.vision.datasets.flowers: https://github.com/PaddlePaddle/Paddle/blob/develop/python/paddle/dataset/flowers.py
mnist: https://github.com/PaddlePaddle/Paddle/blob/develop/python/paddle/dataset/mnist.py

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[wanghaoshuang]

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