Flax/Jax documentation (#8331)

* First addition of Flax/Jax documentation

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>

* make style

* Ensure input order match between Bert & Roberta

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>

* Install dependencies "all" when building doc

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>

* wraps build_doc deps with ""

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>

* Addressing @sgugger comments.

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>

* Use list to highlight JAX features.

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>

* Make style.

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>

* Let's not look to much into the future for now.

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>

* Style

Co-authored-by: Lysandre <lysandre.debut@reseau.eseo.fr>

docs/source/model_doc/bert.rst

@@ -188,3 +188,10 @@ TFBertForQuestionAnswering
 
 .. autoclass:: transformers.TFBertForQuestionAnswering
     :members: call
+
+
+FlaxBertModel
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: transformers.FlaxBertModel
+    :members: __call__

docs/source/model_doc/roberta.rst

@@ -146,3 +146,10 @@ TFRobertaForQuestionAnswering
 
 .. autoclass:: transformers.TFRobertaForQuestionAnswering
     :members: call
+
+
+FlaxRobertaModel
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: transformers.FlaxRobertaModel
+    :members: __call__