Add option to write nemo model as loose files from megatron_ckpt_to_nemo.py
# What does this PR do ?

Adds a command-line flag to megatron_ckpt_to_nemo.py which enables writing output as loose files instead of a .nemo archive.

**Collection**: nlp

# Changelog 
- Adds `--no_pack_nemo_file` argument to megatron_ckpt_to_nemo.py
- If passed, `pack_nemo_file` is set to false in the NLPSaveRestoreConnector used to write the model to disk
- The path given to `--nemo_file_path` is extended before passing to `save_to` so that the model will be written under the given directory

# Usage

```bash
python examples/nlp/language_modeling/megatron_ckpt_to_nemo.py \
  --model_type=gpt \
  --checkpoint_folder=/path/to/checkpoints \
...
  --nemo_file_path=/path/to/output_directory
  --no_pack_nemo_file
```

# Jenkins CI
To run Jenkins, a NeMo User with write access must comment `jenkins` on the PR.

# 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 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)
