vae training and autovae
CI passed: https://github.com/NVIDIA/NeMo/actions/runs/12553124376 @pablo-garay @ethanhe42 thanks!
# What does this PR do ?

this PR introduces VAE training into NeMo that allows users to customize a 16x or 8x reduction VAE on their in house data. Besides, we also introduce a feature that automatically designs VAE based on the GPU memory and latency requirements.

**Collection**: NeMo/Diffusion/VAE

# Changelog 
- it is a new feature, no touch of existing codes.

# Usage
* Please follow the readme to setup the training. 

# GitHub Actions CI

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# Before your PR is "Ready for review"
**Pre checks**:
- [Yes] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [No] Did you write any new necessary tests?
- [Yes] Did you add or update any necessary documentation?
- [No] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [No] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x] New Feature