Add support for Apex distributed Adam optimizer with GPT-3
# What does this PR do ?

Adds support for training GPT-3 with the [Apex implementation of the ZeRO optimizer](https://github.com/NVIDIA/apex/blob/master/apex/contrib/optimizers/distributed_fused_adam.py).

**Collection**: NLP

# Changelog 
- Add option for `distributed_fused_adam` optimizer and integrate with GPT-3 training

# Usage

Set optimizer to `distributed_fused_adam` in config file:

https://github.com/NVIDIA/NeMo/blob/23f6a95fb1479f5d2ff7111adbc5720ed3c58e48/examples/nlp/language_modeling/conf/megatron_gpt_config.yaml#L122

~Note that this optimizer is incompatible with the `megatron_amp_O2` option. It internally applies similar optimizations (bf16 params, wgrad kernel fusion), so refactoring may be warranted in the future.~ This optimizer requires the `megatron_amp_O2` option.

# 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?
- [x] 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/1414.
* Depends on https://github.com/NVIDIA/apex/pull/1432
* This is mostly orthogonal to https://github.com/NVIDIA/NeMo/pull/4380. It just requires changing [this line](https://github.com/NVIDIA/NeMo/blob/5bb47b7638645b6b177638fff4e3b3a82b3fd76c/nemo/collections/nlp/models/language_modeling/megatron_gpt_model.py#L335) to:
```if self.megatron_amp_o2 or self.with_distributed_adam:```