FFTNet vocoder implementation

Unofficial Implementation of FFTNet vocode paper.

• implement the model.
• implement tests.
• overfit on a single batch (sanity check).
• linearize weights for eval time.
• measure the run-time on GPU and CPU. (1 sec audio takes ~47 secs) If anyone knows additional tricks from the paper, let me know. So far I asked the authors but nobody returned.
• train on LJSpeech spectrograms.
• distill model as in Parallel WaveNet paper.