!6485 Add amp_level "auto" to _build_eval_network

Merge pull request !6485 from wanyiming/add_auto_to_eval

mindspore/train/model.py

@@ -193,7 +193,7 @@ class Model:
         else:
             if self._loss_fn is None:
                 raise ValueError("loss_fn can not be None.")
-            self._eval_network = nn.WithEvalCell(self._network, self._loss_fn, self._amp_level in ["O2", "O3"])
+            self._eval_network = nn.WithEvalCell(self._network, self._loss_fn, self._amp_level in ["O2", "O3", "auto"])
             self._eval_indexes = [0, 1, 2]
 
         if self._parallel_mode in (ParallelMode.SEMI_AUTO_PARALLEL, ParallelMode.AUTO_PARALLEL):