Fix TypeError: 'torch.device' object is not callable

[leminhnguyen]

As the issue https://github.com/rishikksh20/iSTFTNet-pytorch/issues/1, the line 164 in stft.py was changed to https://github.com/rishikksh20/iSTFTNet-pytorch/blob/e928a6b604033a3857757562af36241f9225adfc/stft.py#L164

But inverse_transform.device() will raise the exception mentioned in the title. So it can be changed to inverse_transform.device to fix the problem.