Fix broadcast_coalesce for empty tensors (#35965)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/35470
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35965
Differential Revision: D20919377
Pulled By: ezyang
fbshipit-source-id: cfbcb35a44507de1c3ed7e0732cfc3b124b9bc0b
