docs: fix docstring signature in `all_reduce_multigpu` (#54665)
Summary:
fixes https://github.com/pytorch/pytorch/issues/43500
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54665
Reviewed By: ezyang
Differential Revision: D27340481
Pulled By: rohan-varma
fbshipit-source-id: d53c36b41dd26c7a791d3674a5b4b67daaadae13
