Created by: zhangting2020

Add is_empty_grad_op for OpTest, which is used to check if the Op is register without gradient. Before this PR, we used the op_check_grad_white_list.py that records the Ops without gradient. However, the op_check_grad_white_list.py needs to be modified manaully. This PR eliminates the cost of manually managing this file.

使用is_empty_grad_op方法判断Op是否注册了反向。那么原来管理未注册反向Op的白名单op_check_grad_white_list.py就不再需要，因此移除该文件，并且删除了对该文件的approval规则。