Add new get_batch() method to ChunkDataset API (#21797)
Summary:
We plan on generating python bindings for C++ ChunkDataset API using the current Pytorch Dataloader class, which must call get_batch() instead of get_batch(size)
This changes doesnt break the current API, just add one more method that will make future extensions easier (WIP)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21797
Differential Revision: D15830522
Pulled By: soumith
fbshipit-source-id: 7208f305b48bf65d2783eaff43ff57a05e62c255
