Optimize NCCL collectives

Provide optimized versions of our custom-implemented NCCL collectives:

- [ ] `Alltoall`
- [ ] `Gather`
- [ ] `Scatter`
- [ ] `Allgatherv`
- [ ] `Alltoallv`
- [ ] `Reduce_scatterv`
- [ ] `Gatherv`
- [ ] `Scatterv`