Flax implementation of DPT
# What does this PR do?

I tried to implement DPT (Dense Prediction with Transformers) in Flax during my free time! 🚀 
By the way it is the first Segmentation and Depth Estimation model implemented in Flax!

Nits/TODOs:
- [x] Figure out how to properly call `BatchNorm` and `Dropout` inside a `Sequential` 
- [ ] Test equivalency tests 
- [ ] Write documentation - For now they're just copy/pasted
Quetions:
- Why the loss is not implemented in `modeling_dpt.py` ? I can probably help on that since I have already implemented the loss for a university project: https://github.com/antocad/FocusOnDepth/blob/master/FOD/Loss.py 
 
cc @NielsRogge @sanchit-gandhi @patil-suraj 
_The documentation is not available anymore as the PR was closed or merged._