pytorch

74621ca9 - Add allgather_base as per our discussion re: ProcessGroup interface. (#31892)

Add allgather_base as per our discussion re: ProcessGroup interface. (#31892) Summary: Introduce ProcessGroup::allgather_base. No implementation yet: plan to add it one PG backend at a time in a follow up. Pull Request resolved: https://github.com/pytorch/pytorch/pull/31892 Test Plan: No functional changes, no tests yet. Differential Revision: D19290739 Pulled By: agolynski fbshipit-source-id: c2f4947d2980995724c539de7c6d97618e1ba11a