[PG NCCL] Provide work obj in postProcess (#100781)
This will be needed for allreduce_sparse so we can set the outputs on
the work object.
Differential Revision: [D45632428](https://our.internmc.facebook.com/intern/diff/D45632428/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100781
Approved by: https://github.com/fegin
