caffe2: add PIPELINE tag (#41482)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41482
This adds a new tag for use with pipeline parallelism.
Test Plan: CI
Reviewed By: heslami
Differential Revision: D22551487
fbshipit-source-id: 90910f458a9bce68f7ef684773322a49aa24494a
