fix some auto code generation bugs (#45232)

* 修复生成动态图代码时，如果输出没有配置名字，会导致下标越界的问题。

* decide forward_return[0] is not none

* 修改反向yaml前向输出只有一个时，未配置名字，那么输出自动生成为out

* modify code style

paddle/fluid/eager/auto_code_generator/final_state_generator/codegen_utils.py

@@ -446,7 +446,13 @@ class FunctionGeneratorBase:
 
         for i in range(len(forward_returns_list)):
             forward_return = forward_returns_list[i]
-            return_name = forward_return[0]
+            if len(forward_return[0]) == 0:
+                if (len(forward_returns_list) == 1):
+                    return_name = "out"
+                else:
+                    return_name = "out_{}".format(i + 1)
+            else:
+                return_name = forward_return[0]
             return_type = forward_return[1]
             return_pos = forward_return[2]