Support stateful dataset (#15096)
Summary:
Currently re-implements the dataloader for stateful datasets. Outstanding work:
- Refactor DataLoader and DataLoader2 to have common base classes and only differ in specifi pieces of logic,
- Figure out how to not duplicate the `MapDataset` logic for stateful vs. non-stateful
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15096
Differential Revision: D13522043
Pulled By: goldsborough
fbshipit-source-id: 08e461ca51783047f11facc4d27dfa2e4f1e4c2a
