[TTS] Remove nested TTS configs
# What does this PR do ?

In a separate PR (https://github.com/NVIDIA/NeMo/pull/6852) it was brought up that TTS is using nested hydra config, which is against NeMo 1 config file per model policy.

This PR removes nested config from our experimental FastPitch and HiFiGan in favor of a flat structure with duplicated code between 22kHz and 44kHz versions.

Our older non-experimental configs are left unchanged for backwards compatibility.

**Collection**: [TTS]

# Changelog 
- Remove nested configuration from FastPitch and HiFiGan configs using new data loader
- Add separate 22kHz and 44kHz examples for each model.
- Moved hifigan_data.yaml config to separate "hifigan_dataset" folder (not sure if that is a good name) now that it does not need to reuse the generator config files. 

# Before your PR is "Ready for review"
**Pre checks**:
- [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?
- [x] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation
