bugfix: only check backend when mode == Collecive (#36758)

* bugfix: only check backend when mode == Collecive

* fix bug

python/paddle/distributed/fleet/launch.py

@@ -334,7 +334,20 @@ def launch_ps(args, distribute_mode):
     return
 
 
+def infer_backend(args):
+    if args.backend != "auto": return
+    if fluid.core.is_compiled_with_cuda():
+        args.backend = 'nccl'
+    elif fluid.core.is_compiled_with_npu():
+        args.backend = 'unknown'
+    elif fluid.core.is_compiled_with_xpu():
+        args.backend = 'bkcl'
+    else:
+        args.backend = 'gloo'
+
+
 def which_distributed_mode(args):
+    infer_backend(args)  # modify the args.backend
     if args.run_mode is not None:
         assert args.run_mode in ["collective", "ps", "ps-heter"]
 
@@ -368,12 +381,9 @@ def which_distributed_mode(args):
 
     if fluid.core.is_compiled_with_cuda():
         accelerators = fluid.core.get_cuda_device_count()
-        args.backend = 'nccl'
     elif fluid.core.is_compiled_with_npu():
-        args.backend = 'unknown'
         accelerators = fluid.core.get_npu_device_count()
     elif fluid.core.is_compiled_with_xpu():
-        args.backend = 'bkcl'
         accelerators = fluid.core.get_xpu_device_count()
     else:
         accelerators = 0
@@ -400,7 +410,6 @@ def which_distributed_mode(args):
 But found args.servers not empty, default use ps mode")
                 return DistributeMode.PS
             else:
-                args.backend = "gloo"
                 return DistributeMode.COLLECTIVE
         else:
             logger.warning(
@@ -583,20 +592,21 @@ def launch():
     _print_arguments(args)
 
     if args.backend == 'auto':
-        distribute_mode = which_distributed_mode(args)
-        assert args.backend in [
-            'gloo', 'nccl', 'bkcl', 'unknown'
-        ]  # which_distributed_mode must modify args.backend
+        distribute_mode = which_distributed_mode(
+            args)  # which_distributed_mode must modify args.backend
     else:
         assert args.run_mode == 'collective' or args.run_mode == None, "When backend is not 'auto', run mode must be collective"
         check_backend(args.backend)
         distribute_mode = DistributeMode.COLLECTIVE
 
-    block_windows_and_macos(
-        args.backend)  # raise error when using gloo on windows or macos
+    assert args.backend in ['gloo', 'nccl', 'bkcl', 'unknown']
+
     if args.backend == 'gloo':
         logger.warning("launch start with CPUONLY mode")
 
+    block_windows_and_macos(
+        args.backend)  # raise error when using gloo on windows or macos
+
     if enable_elastic(args, distribute_mode):
         launch_elastic(args, distribute_mode)
         return