Created by: tonyyang-svail

The default value of workspace_size_MB has been set to 4096.

https://github.com/PaddlePaddle/Paddle/blob/a06bec128743d7b036636e53eaefbfe989f0d319/paddle/operators/conv_cudnn_op.cc#L25-L31

workspace_size_MB is of type int. It will overflow at

https://github.com/PaddlePaddle/Paddle/blob/4ecbab42d8831bcc31c7d29092fc5c07f39c318c/paddle/operators/conv_cudnn_op.cu.cc#L114-L116

The similar problem appears at backward.

The test won't fail because workspace of size 0 is still a valid initialization. If we fix this we can get 50% performance improvements on ConvNet on TitanX. https://github.com/PaddlePaddle/Paddle/issues/6321