!8571 revert pynative getnext

From: @chujinjin
Reviewed-by: @kisnwang,@limingqi107
Signed-off-by: @limingqi107

mindspore/train/model.py

@@ -366,8 +366,8 @@ class Model:
         with _CallbackManager(callbacks) as list_callback:
             if not dataset_sink_mode:
                 self._train_process(epoch, train_dataset, list_callback, cb_params)
-            elif context.get_context("device_target") == "CPU":
-                logger.warning("The CPU cannot support dataset sink mode currently."
+            elif context.get_context("device_target") == "CPU" or context.get_context("mode") == context.PYNATIVE_MODE:
+                logger.warning("The CPU or PyNative mode cannot support dataset sink mode currently."
                                "So the training process will be performed with dataset not sink.")
                 self._train_process(epoch, train_dataset, list_callback, cb_params)
             else: