[TTS] FastPitch training tutorial
# Addition of FastPitch training tutorial

Please have one of the following:

* New feature: (short feature description) FastPitch training tutorial.
This PR adds FastPitch tutorial to the same notebook as MixerTTS as both generate spectrogram. This tutorial goes through the data download, preprocess, FastPitch import and training pipeline.
Tutorial was added with careful consideration to make minimal changes to MixerTTS tutorial.

## Before your PR is "Ready for review"
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
- [ ] Did you add or updated any necessary documentation?


If you haven't finished some of the above items you can still open "Draft" PR.


## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.
