Cold

Add ImgAug library.

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## Motivation Supports [ImgAug library](https://github.com/aleju/imgaug) to enrich MMClassification augmentation methods. ## Modification A new class, `Imgaug`, is added in `mmcls/datasets/pipelines/transforms.py`. Similar to the function of 'Albu' class, `Imgaug` could empower...