[TTS] Update TTS tutorials, Simplification of testing Mixer-TTS and FastPitch
# What does this PR do ?

Add a one line overview of what this PR aims to accomplish.

**Collection**: [TTS]

# Changelog 
- Update TTS tutorials
- Simplification of testing Mixer-TTS and FastPitch
- Small fix in FastPitch

**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

Closes #3603 #3627 #3628 #3676 