[WIP][DataLoader] CollateIterableDataset prototype (#48933)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48933
Prototype for CollateIterableDataset.
Move `collate_batch_fn` to BatchIterableDataset
- CollateIterableDataset
- [x] Prototype
- [x] Tests
- BatchIterableDataset
- [x] Prototype
- [x] Tests
- SamplerIterableDataset
- [x] Prototype
- [x] Tests
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D25623635
Pulled By: ejguan
fbshipit-source-id: 99ba077619f672551ac15367baaba985db35a9c2
