Extract pytorch dataset/dataloader from fastai datablock/dataloaders

I know how to create a fastai dataloaders from raw pytorch dataset/dataloader. The siamese tutorial and walkwithfastai is very helpful here. My question is: can I extract out the pytorch dataset from a fastai dataloader/datablock? if so, how? and if so, where in the docs/code can I see how this is built or enabled?

@muellerzr would you have a sample in walkwithfastai? Your tutorials have been very helpful, thanks you.