[BugFix] Removing <unk> tokens from decoding timestamp
Signed-off-by: Taejin Park <tango4j@gmail.com>

# What does this PR do ?

Removing `<unk>` tokens from decoding timestamp.
This solves length mismatching error in audio files contain lots of silence.

**Collection**: [Note which collection this PR will affect]
ASR (speaker tasks)

# Changelog 

in decoder_timestamp_utils.py, this PR is making change to exclude
`<unk>`  tokens for both word and word-timestamp output.

`<unk>` tokens are outputted as " " (blank space) in text hypothesis output, 
and this created mismatch of length since split() command removes all kinds of space 
at the start/end

- Also fixed ASR with diar notebook bugs
- Also changed hard-coded variable to a function argument

# 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)
- [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**:
- [ ] 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 ASR 
