Support GPT MCore models in M-LM->NeMo Conversion Script
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# What does this PR do ?

Add support for MCore GPT models in the Megatron-LM to NeMo model conversion script. 
The script originally supported legacy M-LM -> legacy NeMo conversion. 
The script now supports two more paths: legacy M-LM -> MCore NeMo, and MCore M-LM -> MCore NeMo conversions.

MCore M-LM -> legacy NeMo is not a valid use case and hence not supported.

**Collection**: NLP

# Changelog 
- Add specific line by line info of high level changes in this PR.

# Usage
To convert to MCore NeMo model, add `mcore_gpt=True` in the hparams.yaml file that is the input to this script.
To convert from MCore M-LM model, pass in `--mcore_input` to the script.

# Jenkins CI
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**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**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

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# Additional Information
* Related to # (issue)
