Add out= variants for cuda.comm.broadcast/gather/scatter (#39681)
Summary:
Partially fixes https://github.com/pytorch/pytorch/issues/38911
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39681
Differential Revision: D22161342
Pulled By: mrshenli
fbshipit-source-id: 60295077159b02087823e93bb6ebac9d70adea0a
