Open-source suite enables one-click training of all three XTTS v2 components

After Coqui's shutdown, XTTS v2 lacked official training code for its DVAE tokenizer and HiFi-GAN vocoder, and a fixed vocabulary blocked new scripts. The MPL-2.0 Open-XTTS-Trainer covers all three training stages and dynamically extends the vocabulary for new writing systems. On a T4 GPU, HiFi-GAN mel reconstruction loss fell about 67% over 10 epochs.