test_LayerGrad takes too much time to run

[wangkuiyi]

44/252 Test  #44: test_LayerGrad ..............................***Exception: Other 40.38 sec

[luotao1]

In latest teamcity: https://paddleci.ngrok.io/viewLog.html?buildId=16608&buildTypeId=Paddle_PrCi&tab=buildLog&_focus=8388#_state=68

	176/255 Test #48: test_LayerGrad .............................. Passed 127.72 sec

It costs 2 minutes.

[luotao1]

I will take a look at it to reduce its time.