[WIP][DataLoader] Implement CallableIterableDataset (#50045)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50045
Add **CallableIterableDataset**
Modify **CollateIterableDataset** as another callable
Test Plan: Imported from OSS
Reviewed By: H-Huang
Differential Revision: D26209889
Pulled By: ejguan
fbshipit-source-id: d4773026c1269e43b29a3efb16e36e1865fdd024
