The difference is in that torch.distributed.group.WORLD is a constant that identifies the global process group. All functions prefixed with an underscore are not part of the public API and should not be used if you expect version-to-version compatibility.
For now you can continue to specify process_group=None to make it pick up the default process group. I created https://github.com/pytorch/pytorch/issues/17305 57 to track fixing the issue you mention. It should be transparent, just like all functions in the torch.distributed module.