[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>

utils/documentation_tests.txt

@@ -64,6 +64,7 @@ src/transformers/models/sew_d/modeling_sew_d.py
 src/transformers/models/speech_encoder_decoder/modeling_speech_encoder_decoder.py
 src/transformers/models/speech_to_text/modeling_speech_to_text.py
 src/transformers/models/speech_to_text_2/modeling_speech_to_text_2.py
+src/transformers/models/segformer/modeling_tf_segformer.py
 src/transformers/models/swin/modeling_swin.py
 src/transformers/models/trocr/modeling_trocr.py
 src/transformers/models/unispeech/modeling_unispeech.py