Add X-MOD (#20939)

* Add X-MOD to Readme

* Add documentation for X-MOD

* Implement X-MOD

* Fix formatting of X-MOD docs

* Change signature of X-MOD forward methods to use lang_ids

* Minor changes

* Rebase with main and run make fix-copies

* Make suggested changes to docstrings

* Improve code readability

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

* Fix code style

* Conversion script: Remove asserts and type annotations

* Remove _TOKENIZER_FOR_DOC

* XMOD -> Xmod

* Update copyright note

* Fix doctests

* Fix docstring

* Add integration test for FillMaskPipeline

* Revert "Add integration test for FillMaskPipeline"

This reverts commit 4381eb3b1d0f5d85785f89caba83928e6efa6d1f.

* Add end-to-end integration test for mask fill

* make style

* Rebase with main and make fix-copies

---------

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

docs/source/en/_toctree.yml

@@ -381,6 +381,8 @@
         title: Transformer XL
       - local: model_doc/ul2
         title: UL2
+      - local: model_doc/xmod
+        title: X-MOD
       - local: model_doc/xglm
         title: XGLM
       - local: model_doc/xlm