Move checkpoint consolidation to cpu
# What does this PR do ?

This aims to move the megatron checkpoint to nemo conversion to cpu rather than using pytorch distributed following additions introduced [here](https://github.com/NVIDIA/NeMo/pull/6218).  This is useful for large models where the user may have checkpoints to consolidate but don't want to use the massive amount of gpus just for conversion.

**Collection**: [NLP]

# Changelog 
- Change megatron_ckpt_to_nemo.py to use CPU only

# Usage
```python
python3 megatron_ckpt_to_nemo.py \
     --checkpoint_folder <path_to_PTL_checkpoints_folder> \
     --checkpoint_name <checkpoint_name> \
     --nemo_file_path <path_to_output_nemo_file> \
     --tensor_model_parallel_size <tensor_model_parallel_size>\
     --pipeline_model_parallel_size <pipeline_model_parallel_size> \
     --model_type <model type>
```

# Before your PR is "Ready for review"
**Pre checks**:
- [ ] 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

## 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)
