[Docs] Remove outdated comment for sparse all-reduce (#87018)
https://github.com/pytorch/pytorch/pull/23917 switched to using allgatherv instead of allgather for gloo sparse all-reduce. This PR removes a comment saying to use allgatherv if available since that has already been done.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87018
Approved by: https://github.com/H-Huang
