Moving python allgather_coalesced impl from Py to C
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/28857
Test Plan:
buck test caffe2/test:c10d
buck test caffe2/test:distributed_gloo
Reviewed By: mrshenli
Differential Revision: D18209289
fbshipit-source-id: c5a4c4a1aaa07286a05a7c842dda428eeb46f696
