Update xView dataset to return image1, image2

[robmarkcole]

Summary

#1684 updated OSCD to return ["image1", "image2", "mask"]. This issue is to track other CD datasets that require this change:

• LEVIRCDPlus - DONE
• xview2 - requires change to AugmentationSequential to handle [mask1, mask2]

Rationale

Make consistent interface for change detection models

Implementation

No response

Alternatives

No response

Additional information

No response

[adamjstewart]

Note

Once we have a ChangeDetectionTask, we can remove the tests/datamodules/*.py files for these datasets and test them properly with a real trainer. Related to #978.

[adamjstewart]

Are there any change detection datasets with more than 2 images? Wonder if it would make sense to model this similar to a time-series dataset. @nilsleh is working on this but idk if we've finalized the design yet.

[robmarkcole]

Possibly Spacenet 7 - but broadly speaking change detection is a special case of time series, so an API that extends image1, image2, 3,4 etc makes sense