Make ddp_comm_hook_wrapper a private method. (#44643)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44643
This method is not used anywhere else.
Also formatted the file.
Test Plan: buck test caffe2/test/distributed/algorithms/ddp_comm_hooks:test_ddp_hooks
Reviewed By: pritamdamania87
Differential Revision: D23675945
fbshipit-source-id: 2d04f94589a20913e46b8d71e6a39b70940c1461
