add in layer gpt2 tokenizer (#20421)

* add minimal working gpt2 tokenizer

* graph mode and output equivalence tests working

* not today tensorflow. serialization test passing!

* fix style, documentation, docstrings and all that jazz

* passing consistency checks

* move keras nlp to tf dependencies

* fix tf modeling utils and gpt2 attention to enable compiling

* fix (I hope) keras nlp dependencies

* rever changes on generation

* remove debug prints

* remove redundant tf dummy objects

* add from config, get config and max length settings to address review

* let flake ignore the error on distillation you are welcome

* test from config

* add padding test

* address sgugger review

docs/source/en/model_doc/gpt2.mdx

@@ -138,6 +138,10 @@ A list of official Hugging Face and community (indicated by 🌎) resources to h
 
 [[autodoc]] modeling_tf_outputs.TFSequenceClassifierOutputWithPast
 
+## TFGPT2Tokenizer
+
+[[autodoc]] TFGPT2Tokenizer
+
 ## FlaxGPT2Model
 
 [[autodoc]] FlaxGPT2Model