Neural Machine Translation with Megatron Transformer Models (Tensor Parallel and Tarred Datasets Only)
# What does this PR do ?

Adds support to train Neural Machine Translation models with Megatron Transformer models that support tensor parallelism.

**Collection**: NLP

# Changelog 
- Add a `MegatronNMT` class that inherits from `MegatronLMEncoderDecoderModel` and overrides appropriate functions and calls classmethods from `MTEncDecModel`
- Change appropriate methods in `MTEncDecModel` to classmethods so they can be called in `MegatronNMTModel`.
- Add a config file and training script.

# Usage

```python
examples/nlp/machine_translation/megatron_nmt_training.py -cn aayn_base_megatron model.tensor_model_parallel_size=2
```

# 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
- [ ] 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)
