!9149 Fix bug of moments precision error in pynative

From: @joylvliang
Reviewed-by: @chujinjin,@kisnwang,@jjfeing,@chujinjin
Signed-off-by: @chujinjin

mindspore/_extends/builtin_operations.py

@@ -157,6 +157,8 @@ def tuple_to_array(x):
 
 def stop_gradient(x):
     """Implement `stop_gradient`."""
+    if isinstance(x, Tensor):
+        return Tensor(x.asnumpy())
     return x