pytorch

74ead619 - [2/N] [Dispatchable Collectives] Extract ProcessGroup::Work into a separate class and update references (#83680)

[2/N] [Dispatchable Collectives] Extract ProcessGroup::Work into a separate class and update references (#83680) ### Changes - Move ProcessGroup::Work into its own class and update all the references to it / header includes. #### Motivation In the future PRs we will repurpose ProcessGroup to instead contain a list of Backends (ProcessGroupNCCL/Gloo/UCC) and perform dispatching to them based on tensor type. This change is prevent a circular dependency with ProcessGroup depending on Backend and Backend depending on ProcessGroup::Work. Differential Revision: [D38839212](https://our.internmc.facebook.com/intern/diff/D38839212) Pull Request resolved: https://github.com/pytorch/pytorch/pull/83680 Approved by: https://github.com/kwen2501