[SegFormer] TensorFlow port
This PR adds the SegFormer model in TensorFlow (probably the first Transformer-based segmentation model in TensorFlow for which we have PT weights available). 

## TODOs

- [x] Write the foundation components
- [x] Write the image classification layer
- [x] Write components w.r.t semantic segmentation
- [x] Write the semantic segmentation layer
- [x] Add code examples after `call()` methods where relevant
- [x] Write tests
- [x] Modify other related utilities 
- [x] Create Space to allow users to try out the models (preferably with ONNX to reduce the time?)
- [x] Create a Colab Notebook

The final two points are unrelated to the merging of this PR. 

Cc: @deep-diver
_The documentation is not available anymore as the PR was closed or merged._<|||||>Hey @sayakpaul! The code quality error comes from a new release from `black`. Rebasing on `main` should solve the issue as you'll benefit from https://github.com/huggingface/transformers/pull/17918.<|||||>@Rocketknight1 @sgugger this PR is now ready for your review. Some things to note:

* This is the first segmentation model on the TF side that has pre-trained segmentation checkpoints available. Hopefully, it serves as a good foundation for devs contributing TF segmentation models in the future. 
* @deep-diver and I will work on creating a Space and Colab notebook (for off-the-shelf inference and fine-tuning) to allow users to take advantage of a state-of-the-art segmentation model like this one in TF via `transformers`.  

~@NielsRogge even though the error in the CI is coming from `run_tests_pipelines_tf` it seems like the PT test is what is originating the error. Do you mind taking a look?~<|||||>Yes @Rocketknight1's comments are needed [here](https://github.com/huggingface/transformers/pull/17910#discussion_r910724119) as well. <|||||>Yes, I'm sorry! I went deep on a couple of PRs yesterday and today - one for `datasets`, the other for XLA in `transformers`, and haven't had time to review this properly yet. I'll get to it ASAP, though!<|||||>Thanks, @gante. There were no changes except that. Those were reviewed and approved by @Rocketknight1 and @ydshieh. 