Add autograd test for T.GriffinLim

[yoyololicon]

ref #1414
cc @mthrok

Have incorrect jacobian when doing gradcheck.

[mthrok]

Have incorrect jacobian when doing gradcheck.

Hi @yoyololicon

Thanks for the PR. Maybe the reason why it is failing could be the use of float() in the middle of the for loop.

audio/torchaudio/functional/functional.py

Lines 196 to 207 in 90311ed

	for _ in range(n_iter):
	# Store the previous iterate
	tprev = rebuilt
	# Invert with our current estimate of the phases
	inverse = torch.istft(specgram * angles,
	n_fft=n_fft,
	hop_length=hop_length,
	win_length=win_length,
	window=window,
	length=length).float()

Could you modify the code (just removing .float() is a good start and could be sufficient) so that all the intermediate variables have the same dtype as the input Tensor?

[mthrok]

Looks good. Thank you!