Add ViTMatte
# What does this PR do?

This PR adds the ViTMatte model, an elegant approach to image matting, entirely relying on the Vision Transformer backbone doing the heavy work, with a lightweight head on top.

Here's a Colab notebook showcasing inference: https://colab.research.google.com/drive/1pWTn3Iur-NR2xUIyDE31dBgng_hXjSsn?usp=sharing. 

Fixes #25040.