Add cpWER for evaluation of ASR with diarization
# What does this PR do ?

This PR adds `cpWER` calculation feature to diarization-ASR pipeline. 
`WDER` features are removed from `diarization_utils.py` to keep the code simple.
Additionally, docstrings are added to the functions in `diarization_utils.py`

From this PR on, NeMo multispeaker ASR or ASR-with-diarization will be evaluated by cpWER. 
To evaluate cpWER value, corresponding CTM files are needed.

All diarization related evaluation (DER, cpWER) is now included in collections/utils/metric/der.py.
This is for reusability of the diarization evaluation code

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

# Changelog 

- Added functions regarding `cpWER` calculation
- Added docstings in `diarization_utils.py`
- Changed variable names:
 `total_riva_dict` -> `trans_info_dict`,
 `riva_dict` -> `session_trans_dict`,

- Removed functions and imports regarding `WDER` and word-timestamp calculation.
- DER, cpWER are  now included in collections/utils/metric/der.py.
# Usage
* You can potentially add a usage example below

```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 model name or path to .nemo> \
    diarizer.asr.model_path=<pretrained model name or path to .nemo> \
    diarizer.speaker_embeddings.parameters.save_embeddings=False \
    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)
- [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)
