Switch DTensor to use funcol::all_reduce. (#95804)
This is relanding the troubling part of #95009 that caused a regression.
BC: This changes the signature and semantics of DeviceMesh::all_reduce.
DeviceMesh::all_reduce now uses a functional collective under the hood which makes it more easily traceable.
You no longer need to use CommTensor to get a trace.
all_reduce now is async only and uses AsyncCollectiveTensor to ensure proper stream synchronization.
Signature changed: removed async_op param and changes return type from Optional[Work] to torch.Tensor.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95804
Approved by: https://github.com/fegin
