Support PyTorch >=1.7

[unilight]

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When using PyTorch >=1.7, the following error would occur at the beginning of training:

RuntimeError: stft input and window must be on the same device but got self on cuda:0 and window on cpu

A solution to this is https://github.com/kan-bayashi/ParallelWaveGAN/pull/225. However, this can be easily solved by using PyTorch <1.7, so this is rather a note for future users.

[bigpon]

Hi Unilight, thank you very much!! I will merge the modification into the current repo.