Update Mixer-TTS, FastPitch and TTSDataset
- [x] Update nemo.collections.tts.torch.data.TTSDataset and nemo.collections.tts.torch.data.MixerTTSDataset
    - [x] Rename "duration_prior" to "align_prior_matrix"
    - [x] Add `normalized_text` field 
    - [x] Fix minor bugs
- [x] Update FastPitch 
    - [x] Comment unnecessary code and suggest to remove it (+ need to discuss with @blisc)
    - [x] Refactor config 
    - [x] Add correct setup for text normalization and text tokenization to support new config
    - [x] Fix minor bugs
- [x] Update Mixer-TTS 
    - [x] Refactor config 
    - [x] Add correct setup for text normalization and text tokenization to support new config
    - [x] Fix minor bugs
    - [x] Add test to Jenkinsfile for Mixer-TTS
    - [x] Update documentation for Mixer-TTS and Mixer-TTS-X
    - [x] Add checkpoints from NGC Catalog to code
 - [x] Update HiFi-GAN
    - [x] Add checkpoints from NGC Catalog to code (+ need to discuss with @blisc)
- [x] Add & update notebooks
    - [x] Add `MixerTTS_Training.ipynb` notebook
    - [x] Update `Inference_ModelSelect.ipynb` notebook
    - [x] Fix bugs in `FastPitch_Finetuning.ipynb` notebook
  

Signed-off-by: Oktai Tatanov oktai.tatanov@gmail.com