add torch.stft and torch.fft

They would be super useful as fundamental ops (and autograd ops).

STFT can be implemented in terms of ConvTranspose, see github.com/pseeth/pytorch-stft
 

- [x] stft
- [x] istft
- [x] window functions: bartlett, hann, hamming, blackman
- [x] fft, ifft, irfft, rfft
- [x] fftshift, ifftshift

cc @mruberry