add vae training
# What does this PR do ?

Add VAE training, allowing users to customize VAE compression ratio to 16x or 8x and train the customized VAE with the script.

**Collection**:  
NeMo/collections/diffusion/vae

# Changelog 
- Add VAE training on customized datasets
- Add autoVAE that automatically searches for a good customized VAE
- examples of VAE usage, reconstructed images V.S. original images

![1](https://github.com/user-attachments/assets/08122f5b-2e65-4d65-87d7-eceae9d158fb)
![2](https://github.com/user-attachments/assets/6e805a0d-8783-4d24-a65b-d96a6ba1555d)
![3](https://github.com/user-attachments/assets/aab1ef33-35da-444d-90ee-ba3ad58a6b2d)


# Usage
Please see readme in NeMo/nemo/collections/diffusion/vae

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

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.

# Additional Information
* Related to # (issue)
