[TTS] Fix TTS recipes with PTL 2.0
# What does this PR do ?

Update TTS configs so they run with PTL 2.0

**Collection**: [TTS]

# Changelog 
- Replace _ddp_ with _ddp_find_unused_parameters_true_ for all vocoder recipes, otherwise it fails because there are unused parameters during the alternating generator and discriminator training steps.
- Update training_epoch_end() for AudioCodec to be compatible with new PTL interface.

# 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?
- [ ] 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
