[Flan-UL2] Add-flan-ul2 (#21929)

* add doc and readme

* add model docs

* update toctree and fix copies

* update

* update doc file

* fix

* add FLAN-UL2 to configuration mapping

* fixup

* Apply suggestions from code review

* more clarification

---------

Co-authored-by: younesbelakda <younesbelkada@gmail.com>
Co-authored-by: Younes Belkada <49240599+younesbelkada@users.noreply.github.com>

docs/source/en/_toctree.yml

@@ -283,6 +283,8 @@
         title: ESM
       - local: model_doc/flan-t5
         title: FLAN-T5
+      - local: model_doc/flan-ul2
+        title: FLAN-UL2
       - local: model_doc/flaubert
         title: FlauBERT
       - local: model_doc/fnet