Trainer - deprecate tokenizer for processing_class (#32385)

* Trainer - deprecate tokenizer for processing_class

* Extend chage across Seq2Seq trainer and docs

* Add tests

* Update to FutureWarning and add deprecation version

docs/source/en/tasks/document_question_answering.md

@@ -420,7 +420,7 @@ Finally, bring everything together, and call [`~Trainer.train`]:
 ...     data_collator=data_collator,
 ...     train_dataset=encoded_train_dataset,
 ...     eval_dataset=encoded_test_dataset,
-...     tokenizer=processor,
+...     processing_class=processor,
 ... )
 
 >>> trainer.train()
@@ -489,4 +489,4 @@ which token is at the end of the answer. Both have shape (batch_size, sequence_l
 
 >>> processor.tokenizer.decode(encoding.input_ids.squeeze()[predicted_start_idx : predicted_end_idx + 1])
 'lee a. waller'
-```
+```