[Dy2Stat] Fix ProgramTranslator.save_inference_model API Doc (#24584)

As the title.

python/paddle/fluid/dygraph/dygraph_to_static/program_translator.py

@@ -559,14 +559,14 @@ class ProgramTranslator(object):
 
         Args:
             dirname (str): the directory to save the inference model.
-            feed (list[int], optional): the input variable indices of the saved
-                inference model. If None, all input variables of the
-                ProgramTranslator would be the inputs of the saved inference
-                model. Default None.
-            fetch (list[int], optional): the output variable indices of the
-                saved inference model. If None, all output variables of the
-                TracedLayer object would be the outputs of the saved inference
-                model. Default None.
+            feed (list[int], optional): the indices of the input variables of the
+                dygraph functions which will be saved as input variables in
+                inference model. If None, all input variables of the dygraph function
+                would be the inputs of the saved inference model. Default None.
+            fetch (list[int], optional): the indices of the returned variable of the
+                dygraph functions which will be saved as output variables in
+                inference model. If None, all output variables of the dygraph function
+                would be the outputs of the saved inference model. Default None.
         Returns:
             None
         Examples:
@@ -599,12 +599,12 @@ class ProgramTranslator(object):
                         adam.minimize(loss)
                         net.clear_gradients()
                 # Save inference model.
-                # Note that fetch=[0] means we set 'y' as the inference output.
+                # Note that fetch=[0] means we set 'z' as the inference output.
                 prog_trans = ProgramTranslator()
                 prog_trans.save_inference_model("./dy2stat_infer_model", fetch=[0])
 
-                # In this example, the inference model will be pruned based on input (x) and
-                # output (y). The pruned inference program is going to be saved in the folder
+                # In this example, the inference model will be pruned based on output (z).
+                # The pruned inference program is going to be saved in the folder
                 # "./dy2stat_infer_model" and parameters are going to be saved in separate
                 # files in the folder.
         """