[TTS] Audio codec fixes
# What does this PR do ?

A few minor fixes for the audio codec training recipe.

Most important, moving the zero_gradient() call back before the loss calculations. Having it after `optim_disc.step()` results in it saving discriminator gradients during the generator `self.manual_backward(loss_gen_all)` calculation which stops the discriminator from learning (adversarial and feature losses quickly become 0).

**Collection**: [TTS]

# Changelog 
- Zero gradient before discriminator loss calculation
- Change default mel loss scale from 1.0 to 5.0 as it significantly improves ViSQOL and makes training more consistent.
- Fix name of validation time domain loss

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**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
