Debug support for interleaved pipeline parallelism with the distributed Adam optimizer
Signed-off-by: Tim Moon <tmoon@nvidia.com>

# What does this PR do ?

Debug support for interleaved pipeline parallelism with the distributed Adam optimizer.

@erhoo82 has observed poor overlapping of gradient synchronizations and backward compute when running with interleaved pipeline parallelism. This is mostly fixed in Apex with https://github.com/NVIDIA/apex/pull/1514, which makes some changes to the API. This is mostly because the synchronization behavior is different for pipeline parallelism if the interleaved schedule is enabled or disabled (without interleaving we only want overlapped grad syncs in the first pipeline stage, while with interleaving we want overlapped grad syncs in all pipeline stages).

**Collection**: NLP

# Changelog 
- Use updated Apex API for pipeline parallelism, namely for overlapped grad syncs

# Usage

Set the optimizer to `distributed_fused_adam` in the config file:
https://github.com/NVIDIA/NeMo/blob/232db1513259a5c3fcd5689b0f6b89c212f83d03/examples/nlp/language_modeling/conf/megatron_gpt_config.yaml#L189

Enable interleaved pipeline parallelism by setting `virtual_pipeline_model_parallel_size` to a value greater than 1 in the config file:
https://github.com/NVIDIA/NeMo/blob/232db1513259a5c3fcd5689b0f6b89c212f83d03/examples/nlp/language_modeling/conf/megatron_gpt_config.yaml#L51

# 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)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [ ] 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.

Pinging @ericharper and @erhoo82.

# Additional Information
* Depends on https://github.com/NVIDIA/apex/pull/1514
* Follow-up to https://github.com/NVIDIA/NeMo/pull/4900 and https://github.com/NVIDIA/NeMo/pull/5025