[DataPipe] Optimize Grouper from N^2 to N (#68647)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68647
Fixes #68539
When all data from source datapipe depletes, there is no need to yield the biggest group in the buffer.
Test Plan: Imported from OSS
Reviewed By: jbschlosser
Differential Revision: D32562646
Pulled By: ejguan
fbshipit-source-id: ce91763656bc457e9c7d0af5861a5606c89965d5
