adding python all_gather coalesced functionality and testing. (#28634)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28634
caveat 1: this only works in sync mode.
caveat 2: this is going to go away and be replaced by c++ implementation
Test Plan: buck test caffe2/test:distributed_gloo -- test_all_gather_coalesced
Reviewed By: mrshenli
Differential Revision: D18123422
fbshipit-source-id: cfb9950d5d54c6181a5240e7cc9fed88ed47f5d9
