[SegFormer] TensorFlow port (#17910)

* add: segformer utils and img. classification.

* add: segmentation layer.

* feat: working implementation of segformer.

* chore: remove unused variable.

* add test, remaining modifications.

* remove: unnecessary files.

* add: rest of the files.

Co-authored-by: matt <rocketknight1@gmail.com>

* chore: remove ModuleList comment.

* chore: apply make style.

* chore: apply make fixup-copies.

* add  to check_repo.py

* add decode head to IGNORE_NON_TESTED

* chore: run make style.

* chore: PR comments.

* chore: minor changes to model doc.

* tests: reduction across samples.

* add a note on the space.

* sort importats.

* fix: reduction in loss computation.

* chore: align loss function with that of NER.

* chore: correct utils/documentation_tests.txt

Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>

* chore: simplify the interpolation of logits in loss computation.

* chore: return transposed logits when return_dict=False.

* chore: add link to the tf fine-tuning repo.

* address pr comments.

* address niels's comments.

* remove from_pt=True since tf weights are in.

* remove comment from pt model.

* address niels's comments.

Co-authored-by: matt <rocketknight1@gmail.com>
Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>

docs/source/en/index.mdx

@@ -278,7 +278,7 @@ Flax), PyTorch, and/or TensorFlow.
 |          RetriBERT          |       ✅       |       ✅       |       ✅        |         ❌         |      ❌      |
 |           RoBERTa           |       ✅       |       ✅       |       ✅        |         ✅         |      ✅      |
 |          RoFormer           |       ✅       |       ✅       |       ✅        |         ✅         |      ✅      |
-|          SegFormer          |       ❌       |       ❌       |       ✅        |         ❌         |      ❌      |
+|          SegFormer          |       ❌       |       ❌       |       ✅        |         ✅         |      ❌      |
 |             SEW             |       ❌       |       ❌       |       ✅        |         ❌         |      ❌      |
 |            SEW-D            |       ❌       |       ❌       |       ✅        |         ❌         |      ❌      |
 |   Speech Encoder decoder    |       ❌       |       ❌       |       ✅        |         ❌         |      ✅      |