Enable cudnn by default when avaialable in the new back-end

theano/sandbox/gpuarray/__init__.py

@@ -26,7 +26,7 @@ from .type import (GpuArrayType, GpuArrayVariable, GpuArrayConstant,
                    GpuArraySharedVariable, gpuarray_shared_constructor,
                    reg_context, get_context, ContextNotDefined)
 from .basic_ops import as_gpuarray_variable
-from . import opt, nerv
+from . import dnn, opt, nerv
 
 def transfer(x, target):
     try: