Add option to merge distributed optimizer buckets
@ericharper 
Can you please review and merge?
# What does this PR do ?

Adds an option to merge distributed optimizer buckets with GPT. By default each bucket corresponds to a Transformer layer (without interleaved pipeline parallelism) or to virtual pipeline parallel stage (with interleaved pipeline parallelism)

**Collection**: NLP

# Changelog 
- Add option to merge distributed optimizer buckets

# 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 the distributed optimizer with `model.optim.name=distributed_fused_adam` and merge buckets with `model.distributed_adam_bucket_merge_size=<N>`.

For example, if training GPT without pipeline parallelism, `model.distributed_adam_bucket_merge_size=2` will cause each distopt bucket to contain the weights of two Transformer layers.

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- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
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**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

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[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

Pinging @erhoo82.

# Additional Information

