[Scripts] Remove fixed seed for adding noise
<!-- pylint-output-strict-mode-false -->

beep boop 🤖: 🙏 The following files have warnings. In case you are familiar with these, please try helping us to improve the code base.

---

Your code was analyzed with PyLint. The following annotations have been identified:

```
************* Module add_noise
scripts/dataset_processing/add_noise.py:42:0: C0116: Missing function or method docstring (missing-function-docstring)
scripts/dataset_processing/add_noise.py:46:0: C0116: Missing function or method docstring (missing-function-docstring)
scripts/dataset_processing/add_noise.py:76:0: C0116: Missing function or method docstring (missing-function-docstring)
scripts/dataset_processing/add_noise.py:114:0: C0116: Missing function or method docstring (missing-function-docstring)
scripts/dataset_processing/add_noise.py:131:0: C0116: Missing function or method docstring (missing-function-docstring)
scripts/dataset_processing/add_noise.py:28:0: W0611: Unused import random (unused-import)

-----------------------------------
Your code has been rated at 9.37/10
```

---

Thank you for improving NeMo's documentation!
# What does this PR do ?

This PR removes a fixed value of seed when adding noise examples.
With a fixed seed, each worker will select the same noise file.
If required, fixed seed can be set from CLI.

**Collection**: Scripts

# Changelog 
- Changed default seed from `42` to `None`

# Usage
If necessary, the original behavior can be reproduced using

```python
python add_noisy.py ... --seed 42
```

# GitHub Actions CI

The Jenkins CI system has been replaced by GitHub Actions self-hosted runners.

The GitHub Actions CI will run automatically when the "Run CICD" label is added to the PR.
To re-run CI remove and add the label again.
To run CI on an untrusted fork, a NeMo user with write access must first click "Approve and run".

# 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**:
- [ ] 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 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)
