Checkpoint didn't support list output

It actually didn’t work. It sure can input many input as the *args, but list output causes error. For example, my output was like,

return [tensor1, tensor2, tensor3, tensor4, tensor4]

and the error would be like,

TypeError: CheckpointFunctionBackward.forward: expected Variable (got list) for return value 0

I hope I am using it right. :face_with_monocle: