[c10d] Template computeLengthsAndOffsets() (#42706)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42706
Different backends accept different type of length to, like MPI_Alltoallv, nccSend/Recv(), gloo::alltoallv(). So to make computeLengthsAndOffsets() template
Test Plan:
Sandcastle
CI
HPC: ./trainer_cmd.sh -p 16 -n 8 -d nccl
Reviewed By: osalpekar
Differential Revision: D22961459
fbshipit-source-id: 45ec271f8271b96f2dba76cd9dce3e678bcfb625
