Bug fix for Issue #4059, word time stamps for single word cases
Signed-off-by: Taejin Park <tango4j@gmail.com>

# What does this PR do ?

This PR fixes the bug reported in Issue #4059. 
Before this fix, QuartzNet ASR Model could not generate word timestamps for an utterance with a single word.

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

ASR

# Changelog 

- Function named get_word_ts_from_spaces() has been changed to handle the cases where there is only one word in the decoded output.
- Few changes in docstrings.


# Usage

Please refer to [README](https://github.com/NVIDIA/NeMo/blob/d4bf4ed37d9465baa5a83dbf824864b0081c6098/examples/speaker_tasks/diarization/README.md)

```python
python offline_diarization_with_asr.py \
    diarizer.manifest_filepath=<path to manifest file> \
    diarizer.out_dir='demo_asr_output' \
    diarizer.speaker_embeddings.model_path=<pretrained modelname or path to .nemo> \
    diarizer.asr.model_path=<pretrained modelname or path to .nemo> \
    diarizer.asr.parameters.asr_based_vad=True
```

# 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?

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.
