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/wav2vec2.mdx

@@ -62,7 +62,6 @@ This model was contributed by [patrickvonplaten](https://huggingface.co/patrickv
     - save_pretrained
     - batch_decode
     - decode
-    - as_target_processor
 
 ## Wav2Vec2ProcessorWithLM
 
@@ -73,7 +72,6 @@ This model was contributed by [patrickvonplaten](https://huggingface.co/patrickv
     - save_pretrained
     - batch_decode
     - decode
-    - as_target_processor
 
 ## Wav2Vec2 specific outputs