[Fix] AudioCodecModel training fails with PTL 2.0
Currently, `AudioCodecModel` training fails on `main` due to changes in PTL 2.0.

# What does this PR do ?

This PR is fixing two issues when using PTL 2.0.

**Collection**: TTS

# Changelog 
- Support for `training_epoch_end` has been removed in v2.0.0, and we use `on_train_epoch_end`
  - Reference: https://github.com/Lightning-AI/lightning/pull/16520
- Enable the detection of unused parameters in DDP, either by setting the string value `strategy='ddp_find_unused_parameters_true'`
  - Reference: https://github.com/Lightning-AI/lightning/pull/16520

# Usage

N/A

# 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

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


## Who can review?

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[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Related to #6433 
