!8947 [MixedPrecision]Add float64 of mixed_precision_cast

From: @chenfei52
Reviewed-by: @zhunaipan,@zh_qh
Signed-off-by: @zh_qh

mindspore/_extends/builtin_operations.py

@@ -20,6 +20,7 @@ from mindspore.common.tensor import Tensor
 import mindspore.common.dtype as mstype
 from mindspore.common.dtype import dtype_to_nptype, get_py_obj_dtype
 
+
 def scalar_add(x, y):
     """Implement `scalar_add`."""
     return x + y
@@ -164,8 +165,9 @@ hyper_map = C.HyperMap()
 
 def mixed_precision_cast(dst_type, x):
     """Implement `mixed_precision_cast`."""
+
     def cast_inner(data):
-        if isinstance(data, Tensor) and data.dtype in (mstype.float32, mstype.float16):
+        if isinstance(data, Tensor) and data.dtype in (mstype.float32, mstype.float16, mstype.float64):
             return F.cast(data, dst_type)
         return data