NFA updates
# What does this PR do ?

NFA updates: improve API, reduce memory consumption

**Collection**: [Note which collection this PR will affect]
Tools

# Changelog 
Numerous adjustments, including:
- Reduced memory requirements of viterbi decoding
- Removed need to specify model downsample factor
- Now ASS format files can also be created

# Usage
No more need to specify model downsample factor, so can use as follows:

```python
python <path_to_NeMo>/tools/nemo_forced_aligner/align.py \
        pretrained_name="stt_en_citrinet_1024_gamma_0_25" \
        manifest_filepath=<path to manifest of utterances you want to align> \
        output_dir=<path to where your ctm files will be saved> 
```

# 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)
- [x] Did you write any new necessary tests?
- [x] 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**:
- [x] New Feature
- [ ] 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. 
[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)
