fix trainer_pass.py (#37779)

python/paddle/fluid/incubate/fleet/parameter_server/ir/trainer_pass.py

@@ -771,7 +771,7 @@ def find_heter_ops(program, default_device="cpu"):
 
                 """
                 output_vars_no_grad = []
-                for key in pre_op.output_names:
+                for key in op.output_names:
                     for varname in op.output(key):
                         if varname == "@EMPTY@":
                             continue