fsdp support create hybrid-sharded process group for custom backend (#100622)
FSDP creates communication groups for intra-node communication through dist.new_subgroups. Previously, dist.new_subgroups only supported creation based on the number of CUDA devices. However, issue #99706 removed the avaliable-check for CUDA devices, allowing for custom backend create group based on num of custom devices per node.
This PR allows FSDP to explicitly pass device num within the node when creating communication groups for intra-node communication, instead of defaulting to the number of CUDA devices.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100622
Approved by: https://github.com/awgu
