TTS Collection Cleanup
This pull request **introduces 10 alerts** when merging 14140663268bf215e0b77adf3219ae2e9aefa45f into 4561fbfc91c2ffe80be55fdbe9edc9611bd0b2c0 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-ef9bb12c9428d747aca58bd86b42637053f34682)

**new alerts:**

* 10 for Unused import This pull request **introduces 5 alerts** when merging 2c1f807edd54d2da6f9b1953bf9ef1b338f935a1 into 4561fbfc91c2ffe80be55fdbe9edc9611bd0b2c0 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-e1e17319697e68bb4a9fd6847c52daa12889337a)

**new alerts:**

* 5 for Unused import
Updates all TTS to new `_target_` hydra config.
Improve tutorial.ipynb to use parent classes
Make a better OperationMode setter for tts models
Update TTS docs