Opt in flax and tf (#17388)

* initial commit

* add init file

* update globakl init

* update index and dummy objects

* style

* update modelling auto

* fix initi typo in src/transformers

* fix typo in modeling tf auto, opt was in wrong mapping name

* fixed a slow test : saved_model

* style

* fix positionnal embedding if no position id is provided

* update tf test

* update test flax requirements

* fixed serialization

* update

* update tf name to allow smooth convertion

* update flax tests

* style

* fix test typo

* fix tf typo test

* add xla for generate support in causal LM

* fixed bug

* cleaned tf tests

* style

* removed from PT for slow tests

* fix typp

* opt test as slow

* trying to fix GPT2 undefined

* correct documentation and add to test doc

* update tf doc

* fix doc

* fake commit

* Apply suggestions from code review

Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>

* update test based on review

* merged main layer for functionning test

* fixup + quality

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* update long comment

* make fix copies

Co-authored-by: Arthur <arthur@huggingface.co>
Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

docs/source/en/index.mdx

@@ -239,7 +239,7 @@ Flax), PyTorch, and/or TensorFlow.
 |        Nystromformer        |       ❌       |       ❌       |       ✅        |         ❌         |      ❌      |
 |         OpenAI GPT          |       ✅       |       ✅       |       ✅        |         ✅         |      ❌      |
 |        OpenAI GPT-2         |       ✅       |       ✅       |       ✅        |         ✅         |      ✅      |
-|             OPT             |       ❌       |       ❌       |       ✅        |         ❌         |      ❌      |
+|             OPT             |       ❌       |       ❌       |       ✅        |         ✅         |      ✅      |
 |           Pegasus           |       ✅       |       ✅       |       ✅        |         ✅         |      ✅      |
 |          Perceiver          |       ✅       |       ❌       |       ✅        |         ❌         |      ❌      |
 |           PLBart            |       ✅       |       ❌       |       ✅        |         ❌         |      ❌      |