Fix device_mesh test to compatible with old pytorch (#87)

Summary:
Pull Request resolved: #87

device_mesh test has to be run with old pytorch, which does not have GroupName. Import GroupName will break the github oss ci: https://github.com/meta-pytorch/torchcomms/commits/main/. Let's make sure it works with old pytorch

Reviewed By: YulunW

Differential Revision: D88823652

fbshipit-source-id: ce4b00bb00eed51c21393621557259c3211b5758

Author: tanquer

comms/torchcomms/device_mesh.py

@@ -9,10 +9,16 @@
 
 from torch.distributed.device_mesh import _mesh_resources
 
-from torch.distributed.distributed_c10d import GroupName
-
 from torchcomms._comms import _BackendWrapper, _get_store, new_comm, TorchComm
 
+try:
+    from torch.distributed.distributed_c10d import GroupName
+except ImportError:
+    print("GroupName is not available.")
+    # Fallback: GroupName is effectively just str when not available from torch
+    # We use cast to satisfy type checkers while keeping runtime behavior simple
+    GroupName = str  # type: ignore[misc]
+
 
 def _create_torchcomm_process_group(
     comm: TorchComm,