Adding new features and speed up for multi-speaker data simulator
# What does this PR do ?

This PR adds a few features to the existing multispeaker data simulator

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

# Changelog 

- Added a feature to make 1 speaker sessions
- 50-100x speed up by saving loaded wav to dictionary (hashtable) ( 2000ms/session to 20-50ms/session)
- Parallel processing has been added for speed up (up to num_workers=12, it speeds up)
- A few minor bug fixes

# Usage


```python
python multispeaker_simulator.py --config-path='conf' --config-name='data_simulator.yaml' \
  data_simulator.random_seed=42 \
  data_simulator.manifest_filepath=./train-clean-100-align.json \
  data_simulator.outputs.output_dir=./test_rir \
  data_simulator.background_noise.add_bg=True \
  data_simulator.background_noise.background_manifest=./bg_noise.json
  data_simulator.rir_generation.use_rir=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**:
- [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 ASR

# Additional Information
* Related to # (issue)
