Add DPT Flax
# 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 on the library!

Nits/TODOs:
- [x] Figure out how to properly call `BatchNorm` and `Dropout` inside a `Sequential` 
- [x] Deal correctly with `Sequential` layers 
- [x] 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 

All the keys match now but the equivalency test does not pass with `1e-5` but `1e-4` instead<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17779). All of your documentation changes will be reflected on that endpoint.<|||||>Would be great to also incorporate the updates of #17731 <|||||>The Flax model finally predicts the correct depths for the cats (left is Flax and right is Pytorch)! 
![Screenshot 2022-06-25 at 20 12 08](https://user-images.githubusercontent.com/49240599/175785580-bd9b3b25-cc90-4ad0-a262-fbf335e1351f.png)
For that it appears that the transpose conv does not give the same result as Pytorch's implementation that uses a gradient based operation. I fixed it by creating a custom function based on this PR: https://github.com/google/jax/pull/5772 the PR does not seem to be merged soon. We can probably go for this hack for now until the PR in JAX gets merged 
<|||||>As wee discussed, it seems that `align_corners` set to  `False` for both model would not require lowering the tolerance in one of the cases right? 
<|||||>@ArthurZucker exact. I have put a new attribute in the `DPTConfig` and modified a bit the original modeling code but should not break backward compatibility. Now all tests pass with a tolerance of `1e-5` <|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>Thank you very much @sanchit-gandhi for the very detailed review! I had a second round of refactoring while catching up on Flax projects and would love to have a second round of review (left also some unresolved comments) 💪 Thanks again 🙏 
<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.