Cherry-pick recent distopt commits
# What does this PR do ?

This PR cherry-picks several recent distopt commits from the `main` branch into `r1.17.0`. 

**Collection**: NLP

# Changelog 
- GPT support for BF16 grad reductions (#5920)
- Add custom functions to launch distopt communication in interleaved pipeline parallelism (#6183)
- Bugfix for BF16 grad reductions with distopt (#6340)

# Usage

Set the optimizer to `distributed_fused_adam` in the config file:

https://github.com/NVIDIA/NeMo/blob/aaa0ccacf5b4d204ed18f6052e55ff6f20575996/examples/nlp/language_modeling/conf/megatron_gpt_config.yaml#L207

Configure the optimizer with `grad_sync_dtype: bf16`.

# 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
- [ ] 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
