Revert "fix backend check (#2652)" (#2669)

This reverts commit 2fc48c7.

src/accelerate/state.py

@@ -730,28 +730,27 @@ def _prepare_backend(
                 elif is_npu_available():
                     backend = "hccl"
                     distributed_type = DistributedType.MULTI_NPU
-        if (
+        if backend is None and (
             int(os.environ.get("LOCAL_RANK", -1)) != -1
             or get_int_from_env(["PMI_SIZE", "OMPI_COMM_WORLD_SIZE", "MV2_COMM_WORLD_SIZE", "WORLD_SIZE"], 1) > 1
         ):
             if not cpu and is_xpu_available():
                 distributed_type = DistributedType.MULTI_XPU
             else:
                 distributed_type = DistributedType.MULTI_CPU
-            if backend is None or backend == "ccl":
-                if is_ccl_available() and (
-                    get_int_from_env(["CCL_WORKER_COUNT"], 0) > 0 or distributed_type == DistributedType.MULTI_XPU
-                ):
-                    if get_ccl_version() >= "1.12":
-                        import oneccl_bindings_for_pytorch  # noqa: F401
-                    else:
-                        import torch_ccl  # noqa: F401
-
-                    backend = "ccl"
-                elif torch.distributed.is_mpi_available():
-                    backend = "mpi"
+            if is_ccl_available() and (
+                get_int_from_env(["CCL_WORKER_COUNT"], 0) > 0 or distributed_type == DistributedType.MULTI_XPU
+            ):
+                if get_ccl_version() >= "1.12":
+                    import oneccl_bindings_for_pytorch  # noqa: F401
                 else:
-                    backend = "gloo"
+                    import torch_ccl  # noqa: F401
+
+                backend = "ccl"
+            elif torch.distributed.is_mpi_available():
+                backend = "mpi"
+            else:
+                backend = "gloo"
         if distributed_type is None:
             distributed_type = DistributedType.NO
         return backend, distributed_type