Distributed optimizer support for experimental FP8 tensors
This PR is stale because it has been open for 14 days with no activity. Remove stale label or comment or update or this will be closed in 7 days. This PR was closed because it has been inactive for 7 days since being marked as stale.
# What does this PR do ?

This PR integrates with Transformer Engine's experimental `Float8Tensor`s. This allows the model to only store FP8 weight matrices. The distributed optimizer stores an FP32 master copy of the weights and performs param all-gathers in FP8.

**Collection**: NLP

# Changelog 
- Adds distributed optimizer support for FP8 parameters
- Adds the option to initialize GPT with FP8 parameters

# Usage

Run GPT, e.g. with the config at https://github.com/NVIDIA/NeMo/blob/main/examples/nlp/language_modeling/conf/megatron_gpt_config.yaml.

Enable FP8 support with `model.fp8=True`, FP8 parameters with `model.fp8_params=True`, and the distributed optimizer with `model.optim.name=distributed_fused_adam`.

# 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?
- [x] 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
- [ ] 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
* Depends on https://github.com/NVIDIA/apex/pull/1749
* Uses Transformer Engine `Float8Tensor` added in https://github.com/NVIDIA/TransformerEngine/pull/452
* Closes https://github.com/NVIDIA/NeMo/pull/7469 and https://github.com/NVIDIA/NeMo/pull/7565