Update error message (#121644)
Summary:
We don't want people to move to NCCL exp without explicit opt in. It seems that sparse allreduce was accidentally called and people were confused whether they should use NCCL exp instead.
Update the error message to explicitly say that sparse_allreduce is not supported.
Test Plan: sandcastle
Differential Revision: D54759307
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121644
Approved by: https://github.com/awgu
