* add 4 retinal vessel segmentation configs of UNet

* fix flip augmentation

* add unet benchmark on 4 medical datasets

* fix hrf bug

* add RandomRotate for transforms

* change rotation function to mmcv.imrotate

* refactor

* add unittest

* fixed test

* fixed docstring

* fixed test

* add more test

* fixed repr

* rename to prob

* fixed unittest

Co-authored-by: hkzhang95 <GodBlessZhk@outlook.com>