[WIP][DataLoader] Prototype of BatchIterableDataset (#49186)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/49186
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D25623636
Pulled By: ejguan
fbshipit-source-id: 01a08cccb69301481c55b46358203354b9b4f5fa
