Add ability to override _reduce_ex_ function of DataPipe (#52858)
Summary:
Required for `torchdata` graph functions
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52858
Reviewed By: H-Huang
Differential Revision: D26736348
Pulled By: VitalyFedyunin
fbshipit-source-id: 1735e88374090422e6365d07d5b84075e371500c
