Make NFA subtitles stay until end of video
# What does this PR do ?

Write an extra line at the end of the ASS files which will ensure the subtitles of the final segment are visible until the end of the video. Without this, it means that if we create an MP4 file from an audio file by writing the subtitles on a plain background, at the end of the video we are always left with a blank screen, which is not a nice UX as the text feels to disappear very suddenly.

**Collection**: `tools/nemo_forced_aligner.py`

# Changelog 
- extra code to get the duration of the audio file and write the extra lines at the end of the ASS files.

# Usage
No change in usage.

# 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)
- [ N/A ] Did you write any new necessary tests?
- [ N/A ] Did you add or update any necessary documentation?
- [ N/A ] 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?

Anyone in the community is free to review the PR once the checks have passed. 
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# Additional Information
* Related to # (issue)
