Cleanup scatter-related code (#103074)
This patch cleans up scatter-related code.
GNN-specific implementation for scatter operation uses `radix_sort` to sort the indices, as `radix_sort` was recently moved to FBGEMM common utils (via [pytorch/FBGEMM#1672](https://github.com/pytorch/FBGEMM/pull/1672)), we do not need a local copy of the algorithm anymore.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103074
Approved by: https://github.com/mikaylagawarecki
