conv_cudnn_op not utilizing workspace

[tonyyang-svail]

The default value of workspace_size_MB has been set to 4096.

Paddle/paddle/operators/conv_cudnn_op.cc

Lines 25 to 31 in a06bec1

	AddAttr<int>("workspace_size_MB",
	"workspace size for cudnn, in MB, "
	"workspace is a section of GPU memory which will be "
	"allocated/freed each time the operator runs, larger "
	"workspace size can increase performance but also requires "
	"better hardware. This size should be chosen carefully.")
	.SetDefault(4096);

workspace_size_MB is of type int. It will overflow at

Paddle/paddle/operators/conv_cudnn_op.cu.cc

Lines 114 to 116 in 4ecbab4

	if (user_workspace_size > 0) {
	workspace_size_limit = user_workspace_size * 1024 * 1024;
	}

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. #6321