Added list_available_models
Signed-off-by: Evgeniy Shabalin <36159472+treacker@users.noreply.github.com>

# What does this PR do ?

Added LJSpeech model to list_available_models

**Collection**: [TTS]

# Changelog 
- Add specific line by line info of high level changes in this PR.

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


## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[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 # (issue)
