Megatron MPT-7B Support
# What does this PR do ?

Adds changes to Megatron GPT which supports the MPT-7B architecture

**Collection**: NLP

# Changelog 
- nemo/collections/nlp/models/language_modeling/megatron/gpt_model.py
   + Adds support for the megatron_legacy argument to flow down to CoreAttention
   + Adds a bypass to load_state_dict which lets you load directly into the language_model

- nemo/collections/nlp/models/language_modeling/megatron_base_model.py
   + Fixes an error caused by register_artificat demanding cfg.tokenizer.model when one isn't needed, for HF tokenisers

- nemo/collections/nlp/models/language_modeling/megatron_gpt_model.py
  + Adds support for the megatron_legacy flag
  + Adds an optional override_vocab_size flag (necessary for hard import of HF weights). If this param isn't given, it loads padded_vocab_size as normal

- nemo/collections/nlp/modules/common/megatron/layer_norm_1p.py
  + Added support for the MPT-7B lower precision fp16 layer norm which works without a bias. Unfortunately, the existing lower precision layer norm from apex absolutely requires a bias term

- nemo/collections/nlp/modules/common/megatron/transformer.py
  + Added logic to remove bias terms on layer norm layers

# Usage
* You can restore an MPT-7B model using the MegatronGPTModel class with `restore_from`

# 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
- [X] 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)
