Fluid's vgg16 doesn't converge now.

[gongweibao]

We need #9002 to guarantee this.
Script from https://github.com/dzhwinter/benchmark

Pass = 1, Training performance = 240.092875 imgs/s, Train accuracy = 0.099440, Test accuracy = 0.100400
Pass = 2, Training performance = 240.027211 imgs/s, Train accuracy = 0.099540, Test accuracy = 0.100000
Pass = 3, Training performance = 240.151129 imgs/s, Train accuracy = 0.099580, Test accuracy = 0.100100
Pass = 4, Training performance = 240.201239 imgs/s, Train accuracy = 0.099760, Test accuracy = 0.094900
Pass = 5, Training performance = 240.154561 imgs/s, Train accuracy = 0.098660, Test accuracy = 0.100000
Pass = 6, Training performance = 240.091699 imgs/s, Train accuracy = 0.099620, Test accuracy = 0.100000
Pass = 7, Training performance = 240.322419 imgs/s, Train accuracy = 0.097080, Test accuracy = 0.100000
Pass = 8, Training performance = 239.932700 imgs/s, Train accuracy = 0.098200, Test accuracy = 0.100300
Pass = 9, Training performance = 239.198880 imgs/s, Train accuracy = 0.099780, Test accuracy = 0.099800
Pass = 10, Training performance = 240.219263 imgs/s, Train accuracy = 0.098220, Test accuracy = 0.100000

[gongweibao]

Locate the problem now.