MKL-DNN: Add UT to check whether primitives already exist in backward#15856

luotao1 merged 2 commits intoPaddlePaddle:developPaddlePaddle/Paddle:developfrom

kbinias:kbinias/activation-mkldnn-utkbinias/Paddle:kbinias/activation-mkldnn-utCopy head branch name to clipboard