!10098 fix bug of pynative's mixprecision

From: @lianliguang
Reviewed-by: @kisnwang,@zhoufeng54
Signed-off-by: @zhoufeng54

mindspore/nn/cell.py

@@ -647,7 +647,7 @@ class Cell(Cell_):
                 param.set_cast_dtype(mstype.float32)
             elif self._mindspore_flags.get('fp16'):
                 param.set_cast_dtype(mstype.float16)
-            else:
+            elif hasattr(param, "set_cast_dtype"):
                 # retest dtype
                 param.set_cast_dtype()
         return param