This implements a new `wait_stream` API in Work that matches how `wait` works for ProcessGroupNCCL for CPU based backends such as Gloo.
The idea is to support Gloo communication overlap in FSDPv2/HSDP with minimal changes to FSDP.
There was a previous attempt to make FSDPv2 use Work.wait but given the extensive stream semantics used it doesn't play nicely. https://github.com/pytorch/pytorch/pull/148780
This uses a "Baton" CUDA kernel which spinlocks on a pinned CPU tensor waiting for it to be set.
Test plan:
```
pytest test/distributed/test_c10d_gloo.py -v -k wait_stream
pytest test/distributed/test_c10d_nccl.py -v -k wait_stream
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/156883
Approved by: https://github.com/kwen2501, https://github.com/fduwjj
