[Device] Replace hardcoded devices with 'torch._C._get_accelerator()' (pytorch#139032)

I noticed that some hard-code like `"cuda" if torch.cuda.is_available() else "cpu"` which can be replaced with `torch._C._get_accelerator()`

Pull Request resolved: pytorch#139032
Approved by: https://github.com/ezyang

Author: shink

torch/distributed/_composable/fsdp/_fsdp_init.py

@@ -58,8 +58,8 @@ def _init_default_fully_shard_mesh() -> DeviceMesh:
     if not dist.distributed_c10d.is_initialized():
         dist.distributed_c10d.init_process_group()
     default_pg = dist.distributed_c10d._get_default_group()
-    device_type = "cuda" if torch.cuda.is_available() else "cpu"
-    mesh = init_device_mesh(device_type, mesh_shape=(default_pg.size(),))
+    device = torch._C._get_accelerator()
+    mesh = init_device_mesh(device.type, mesh_shape=(default_pg.size(),))
     return mesh