Created by: wojtuss

In this patch, we fix a bug in reading dataset and add support for reading a whole imagenet dataset preprocessed using the tool from https://github.com/PaddlePaddle/Paddle/pull/16529.

Most changes come from the diff between https://github.com/PaddlePaddle/Paddle/pull/16532 and merged https://github.com/PaddlePaddle/Paddle/pull/16399.

Additional methods are added to tester_helper.h. Could be refactored to reuse existing methods but would require more changes in other tests - left to further refactoring.

test=develop