Rearrange stopping condition in CompositeReader (#20062)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20062
Previously, the batch counter is incremented even if none of the readers has data. In this diff,
1) Limiter is applied to the last reader so that the batch counter is not incremented unless the first N-1 readers have data
2) The stop blob of the last reader as the stop blob of the task so that it's checked before the counter is incremented
Reviewed By: xianjiec
Differential Revision: D15099761
fbshipit-source-id: 47ed6c728118fe453cf57ac3457085867939485b
