[DataLoader] Introduce ConcatMapDataPipe functional datapipe (#61010)
Summary:
As part of https://github.com/pytorch/pytorch/issues/57031, this PR adds the ConcatMapDataPipe functional datapipe for the MapDataPipe class.
We may need to discuss how to treat the datapipes with no valid length. For now, I just use them as if they have infinite length and the `__getitem__` could not go pass them.
Thank you for your time on reviewing this~
cc ejguan
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61010
Reviewed By: soulitzer
Differential Revision: D29587679
Pulled By: ejguan
fbshipit-source-id: 5eb97fa727209bec6c534520057c64a78000626e
