Replace as_target context managers by direct calls (#18325)

* Preliminary work on tokenizers

* Quality + fix tests

* Treat processors

* Fix pad

* Remove all uses of  in tests, docs and examples

* Replace all as_target_tokenizer

* Fix tests

* Fix quality

* Update examples/flax/image-captioning/run_image_captioning_flax.py

Co-authored-by: amyeroberts <amy@huggingface.co>

* Style

Co-authored-by: amyeroberts <amy@huggingface.co>

docs/source/en/model_doc/trocr.mdx

@@ -94,7 +94,6 @@ See the [model hub](https://huggingface.co/models?filter=trocr) to look for TrOC
     - save_pretrained
     - batch_decode
     - decode
-    - as_target_processor
 
 ## TrOCRForCausalLM