[DDP] Rename the member divFactor_ as div_factor for naming consistency in reducer (#59523)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59523
Should use snake case instead of camel case for the consistency.
ghstack-source-id: 130759655
Test Plan: buck test mode/dev-nosan caffe2/test/distributed:distributed_nccl_fork -- test_ddp_grad_div_uneven_inputs
Reviewed By: cbalioglu
Differential Revision: D28922896
fbshipit-source-id: e04298284a78b2e71b562f790a878731962f873a
