Add augmentations which only transform image (e.g. Constrast, Equalize, Color, Brightness). (#3643)

* implements augmentation which only transform images (e.g. Color,Sharpness) in process

* Add augs which only transform image (e.g. Sharpness, Equalize, Color) in process.

* remove test_augonlyimg.py

* re-implement color,brightness,equalize,contrast using mmcv

* rename aug_only_img as img_augment

* remove aug_only_img related files

* merge img_augment.py into auto_augment.py