Moving python allgather_coalesced impl from Py to C. (#29059)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29059
This is a resubmit of reverted diff D18209289 ( PR #28857 ).
Test Plan:
buck test caffe2/test:c10d
buck test caffe2/test:distributed_gloo
Reviewed By: pietern
Differential Revision: D18277097
fbshipit-source-id: aecfd7206d70829f0cac66182bf02fccee410fed
