TTS - Mixing datasets for FastPitch + HiFiGAN
### Discussed in https://github.com/NVIDIA/NeMo/discussions/3678

<div type='discussions-op-text'>

<sup>Originally posted by **gedefet** February 15, 2022</sup>
Hi All.
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At the end of the FastPitch_Finetuning.ipynb tutorial:

https://github.com/NVIDIA/NeMo/blob/main/tutorials/tts/FastPitch_Finetuning.ipynb
<br>
it suggest two different ways to improve performance, one of which is:
<br>
_Mix new speaker data with old speaker data
We recommend to train fastpitch using both old speaker data (LJSpeech in this notebook) and the new speaker data. In this case, please modify the .json when finetuning fastpitch to include speaker information:_
<br>
but it isn't 100% clear how to do that.
<br>
I found another FastPitch_Finetuning.ipynb here: 

https://colab.research.google.com/github/NVIDIA/NeMo/blob/stable/tutorials/tts/FastPitch_Finetuning.ipynb
<br>
Which is different than the other. 
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It seems that it has two functions defined to do the mixing, but it seems that this colab is abandoned. Nevertheless, I think that has very useful code, and that has tackled this part of mixing datasets.
<br>
Is this true? Or do I need something more that is not there?Why it is abandoned?
<br>
Thanks!</div>