Avoid storing extra copy of params in distributed Adam optimizer
# What does this PR do ?

https://github.com/NVIDIA/apex/pull/1463 adds an option to the distributed Adam optimizer that reduces memory usage for BF16 models. Instead of storing FP32 main params in the optimizer, we can just store 16-bit param remainders and reconstruct the FP32 main params from the BF16 model params.

**Collection**: NLP

# Changelog 
- Store 16-bit param remainders in dist Adam instead of full FP32 main params
- Update Dockerfile to build Apex with dist Adam support

# Usage
Set optimizer to `distributed_fused_adam` in config file:
https://github.com/NVIDIA/NeMo/blob/c0bfa6f07f766a3abd1804f5b666474887e0a1e4/examples/nlp/language_modeling/conf/megatron_gpt_config.yaml#L150

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

## 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.

Pinging @ericharper and @yaoyu-33.

# Additional Information
* Depends on https://github.com/NVIDIA/apex/pull/1463
