Yes.
You can fix it pretty easily, though:
https://github.com/SeanNaren/warp-ctc/blob/8719493fbd0dd9b1195d531184d8e8b1d424abe9/pytorch_binding/warpctc_pytorch/ 27init.py#L42
change that to
return self.grads * grad_output, None, None, None

and it’ll probably be fine.