need fastspeech2 customization
### Discussed in https://github.com/NVIDIA/NeMo/discussions/5018

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<sup>Originally posted by **rkrohit14092** September 27, 2022</sup>
How to add additional words in the dictionary of fastspeech2. I want to customize the dictionary as per my use case. 
![image](https://user-images.githubusercontent.com/52836074/192561638-b3c7c51e-3c29-4a86-a926-9e36cc6067c5.png)
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