[BugFix] decoder timestamp count has a mismatch when <unk> is decoded
Signed-off-by: Taejin Park <tango4j@gmail.com>

# What does this PR do ?


decoder timestamp count has a mismatch when <unk> is decoded.
Instead of having a temporary solution to ignore <unk>, the script is changed to 
output <unk> as decoded output with a correct timestamps.

https://github.com/NVIDIA/NeMo/pull/5481/files

In this previous PR, the <unk> symbol is coded to be ignored, 
but it still causes mismatch errors when <unk> is in the middle of two isolated one-token words.

This PR fixes such errors.


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

# Changelog 

`ctc_decoder_predictions_tensor_with_ts()` in decoder_timestamps_utils.py
is changed to output the actual decoded text with the matching word timestamps.

# Usage

```python
python offline_diar_with_asr_infer.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 \
    diarizer.speaker_embeddings.parameters.save_embeddings=False
```

# 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?
- [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**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation


## Who can review?

ASR contributors

# Additional Information
* Related to # (issue)
