speedup image classification processing

[shippingwang]

Speedup Image Classification training speed

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• [x] add eval shape arguments, for different image shape processing, like inception v4
• [x] add multiprocess reader, set cv2 as the default reader
• [x] call reading files func in every training loop, and use pass id as the seed to implement global shuffle
• [x] run normalization operations onto GPU, use uint8 to transfer data in order to use fewer bytes

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Experiments Results: