fix infer device_map (#2105)

Author: Jintao-Huang

swift/llm/infer.py

@@ -141,7 +141,7 @@ def prepare_model_template(args: InferArguments,
     else:
         print(f'device_count: {torch.cuda.device_count()}')
     model_kwargs = {}
-    if device_map is not None:
+    if device_map is None:
         device_map = get_default_device_map()
     model_kwargs['device_map'] = device_map
     if device_map == 'auto':