Support distributed Adam with T5 and support overlapped grad reductions with pipeline parallelism
# What does this PR do ?

Generalize distributed Adam support for GPT-3 to T5 and other Megatron-LM models. It also implements several performance optimizations.

**Collection**: NLP
# Changelog 
- When params are BF16, distributed Adam will store 16-bit param remainders instead of FP32 main params
- Decouples distributed Adam support from Megatron O2-level optizations
- Add support for Apex distributed Adam optimizer with other Megatron-LM models, namely T5
- Add support for overlapped grad reductions with pipeline or sequence parallelism

# Usage

Set optimizer to `distributed_fused_adam` in config file:

https://github.com/NVIDIA/NeMo/blob/265f7b17876aef7937cf0fedf7a76885bdc63288/examples/nlp/language_modeling/conf/megatron_t5_config.yaml#L137

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**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?
- [x] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
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**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

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# Additional Information
* Closes https://github.com/NVIDIA/NeMo/pull/4799
* Depends on https://github.com/NVIDIA/apex/pull/1475