make op_lifter work for Op with multiple output.

theano/sandbox/gpuarray/opt.py

@@ -60,7 +60,6 @@ def op_lifter(OP):
         def local_opt(node):
             if type(node.op) in OP:
                 # This does not support nodes that have more than one output.
-                assert len(node.outputs) == 1
                 # either one of our inputs is on the gpu or
                 # all of our client are on the gpu
                 if (any([i.owner and i.owner.op == host_from_gpu
@@ -71,7 +70,9 @@ def op_lifter(OP):
                     # This is needed as sometimes new_op inherit from OP.
                     if new_op and new_op != node.op:
                         if isinstance(new_op, theano.Op):
-                            return [host_from_gpu(new_op(*node.inputs))]
+                            return [host_from_gpu(o) for o in new_op(*node.inputs, return_list=True)]
+                        elif isinstance(new_op, (tuple, list)):
+                            return [host_from_gpu(o) for o in new_op]
                         else:  # suppose it is a variable on the GPU
                             return [host_from_gpu(new_op)]
             return False