The training script requires 4 command line arguments.
`--data` is the path of the training dataset, `--output` is the path of the output direcctory (we recommend to use a subdirectory in `runs` to manage different experiments.)
`--device` should be "cpu" or "gpu", `--nprocs` is the number of processes to train the model in parallel.
Synthesize waveform. We assume the `--input` is a text file, one sentence per line, and `--output` is a directory to save the synthesized mel spectrogram(log magnitude) in `.npy` format. The mel spectrograms can be used with `Waveflow` to generate waveforms.