Speedup the speech commands dataset processing script
Signed-off-by: Shantanu Acharya <shantanua@nvidia.com>

# What does this PR do ?

Improves the speed of the google speech commands dataset processing script by adding multiprocessing support.

**Collection**: dataset processing scripts

# Changelog 
File changed: `scripts/dataset_processing/process_speech_commands_data.py`

Functions added:
- `__construct_filepaths`: Refactored the filepath construction of the audio samples into a separate function
- `__construct_silence_set`: Used for multiprocessing - Constructs the silence set
- `__rebalance_files`: Used for multiprocessing - rebalances the number of files in each class
- `__prepare_metadata`: Used for multiprocessing - create the manifest entry for a particular audio file

Functions modified: `__process_data`
- Accepts a new argument `num_processes` which determines the number of processes to use for multiprocessing
- Refactored the filepath construction, silence set construction, rebalancing and metadata construction logic to separate functions as described above

For writing the silence files, changed `librosa.output` to `soundfile.write` since in newer versions of librosa, the output module has been removed.

# Usage

```python
python process_speech_commands_data.py \
    --data_root=<absolute path to where the data should be stored> \
    --data_version=<either 1 or 2, indicating version of the dataset> \
    --class_split=<either "all" or "sub", indicates whether all 30/35 classes should be used, or the 10+2 split should be used> \
    --num_processes=<number of processes to use for data preprocessing> \
    --rebalance \
    --log
```

# 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

## Who can review?

Anyone in the NeMo.
