Implement wrapper around albumentations

[tobiasvanderwerff]

It would be nice to use the albumentations library in Detectron2. The albumentations library provides a wide range of image augmentations and it is therefore useful to integrate with Detectron2, as an addition to the image augmentations that it already provides.

This feature has already been discussed earlier in this issue from 2021, but, as far as I can see, did not lead to a successful merge.

I am building on the code provided by @KUASWoodyLIN, which is a great start. However, for integration with the default DatasetMapper, it is necessary to convert between the Detectron2 bounding box format and the bounding box format expected by albumentations, which I have added. In addition, I also added a test for the new T.Albumentations class. Here's an example of how it can be used:

from detectron2.data import DatasetMapper
import detectron2.data.transforms as T
import albumentations as A

augs = [
    T.Albumentations(A.HorizontalFlip(p=0.5)),
    T.Albumentations(A.RandomBrightnessContrast(p=0.5)),
    T.FixedSizeCrop(crop_size=(256, 256)),
]

mapper = DatasetMapper(cfg, is_train=True, augmentations=augs)

As shown here, the T.Albumentations class can be used in conjunction with other Detectron2 augmentations.

Some limitations of this implementation:

• It only works for images, bounding boxes, and masks. Keypoint augmentations could be added relatively easily, by implementing the T.AlbumentationsTransform.apply_coords method.
• It does not work for composite transforms, e.g. A.Compose or A.OneOf.

Please let me know if anything is missing or if there are still changes that need to be made.

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