I haven’t tried carvana with the last version of fastai. Note that we can’t help you with just
RuntimeError: CUDA error: device-side assert triggered
This is a generic cuda error due to a bad-index problem and you need to try one forward pass on the CPU to get more details on it.
We also can’t help you if you don’t give us the version of fastai you’re working with, since there has been a lot of changes as we refined the data block API (which is stabilized now).
As for using a customized open function for the masks (if you want to set div=True), just change (or subclass) the open method of SegmentationLabelList.