Add zero_mask function for vectorized functions. (#32985)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32985
This can be useful in many situations to decide whether all elements are
zeros or non-zeros, such as elu as shown in #32986 .
Test Plan: Imported from OSS
Differential Revision: D19794549
Pulled By: VitalyFedyunin
fbshipit-source-id: 1be1c863d69b9a19fdcfcdd7cb52343066f740d3
