Update `ema.py` for full Lightning 2.0 and 2.1 support
# What does this PR do ?

Updates `ema.py` for full Lightning 2.0 and 2.1 support, and also adds full docstrings for the EMA `Callback` and its components.

# Changelog 
- Update imports for the `EMA` `Callback`
- Add docstrings to the `EMA` `Callback`

# Usage
* Being able to use the `EMA` `Callback` in Lightning 2.0 as well as Lightning 2.1 models.

# 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?
- [ ] 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
- [x] 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.

