Megatron support
# What does this PR do ?

Add MegatronBert support for more NLP models; 
Add support for finetuning a downstream task which used MegatronBert as pretrained model

Fix RIVA and ONNX export issues of Bert and MegatronBert (This is coming from https://github.com/NVIDIA/NeMo/pull/3883 PR)
The change in order of arguments won't break usage of anyone unless they deliberately turned off parameters check and passed arguments in positional fashion. ONNX export is one of the places that does that internally in PyTorch and that's why we need to make that order consistent across HF and Nemo BERT models and NLPModel that uses those modules as input_module.

NLPModel __init__() refactoring ( https://github.com/NVIDIA/NeMo/pull/3887 ) 
This is done to reduce code duplication and make it manageable. Each class has their own minor config differences and registers own artifacts, and those subtle inconsistencies break assumptions downstream (e.g. in Riva)

**Collection**: [Note which collection this PR will affect]

NLP

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

# Usage
* You can potentially add a usage example below

```python
# Add a code snippet demonstrating how to use this 
```

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**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)
  - [x ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x ] New Feature
- [x ] Bugfix
- [ ] Documentation

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## Who can review?

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[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)
