[NLP] HF -> NeMo conversion enhancements
Closing to follow up elsewhere - see the description above for details.
# Announcement📢📢📢

I'm closing this PR in favor of a series smaller PRs with all the corresponding changes applied.
* https://github.com/NVIDIA/NeMo/pull/7977
* https://github.com/NVIDIA/NeMo/pull/7978
* https://github.com/NVIDIA/NeMo/pull/7979
* https://github.com/NVIDIA/NeMo/pull/7980
* https://github.com/NVIDIA/NeMo/pull/7981
* https://github.com/NVIDIA/NeMo/pull/7982

Hopefully, this makes it easier to review and integrate the changes.

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# What does this PR do ?

This PR improves community models conversion scripts: Llama2, MPT, and StarCoder.

**Collection**: NLP

# 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
- [x] Documentation


## 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.