add float64 cast for pynative auto mixed precision

mindspore/nn/cell.py

@@ -263,11 +263,14 @@ class Cell(Cell_):
         self._attr_synced = False
 
     def _cast_mixed_precision_inputs(self, inputs, dst_type):
+        """Cast input for mixed precision"""
         res = list()
         for item in inputs:
             if isinstance(item, tuple):
                 res.append(self._cast_mixed_precision_inputs(item, dst_type))
-            elif item.dtype in {mstype.float16, mstype.float32}:
+            elif isinstance(item, float):
+                res.append(cast(item, dst_type))
+            elif hasattr(item, "dtype") in {mstype.float16, mstype.float32, mstype.float64}:
                 res.append(cast(item, dst_type))
             else:
                 res.append(item)