Add RWKV-4 (#22797)

* First draft of RWKV-4

* Add support for generate

* Style post-rebase

* Properly use state

* Write doc

* Fix doc

* More math

* Add model to README, dummies and clean config

* Fix init

* multiple fixes:

- fix common tests
- fix configuraion default values
- add CI test for checking state computation
- fix some CI tests

* correct tokenizer

* some tweaks

- fix config docstring
- fix failing tests

* fix CI tests

- add output_attention / output_hidden_states
- override test_initialization
- fix failing CIs

* fix conversion script

- fix sharded case
- add new arguments

* add slow tests + more fixes on conversion script

* add another test

* final fixes

* change single name variable

* add mock attention mask for pipeline to work

* correct eos token id

* fix nits

* add checkpoints

* Apply suggestions from code review

Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

* add `tie_word_embeddings` in docstring

* change tensor name

* fix final nits

* Trigger CI

---------

Co-authored-by: younesbelkada <younesbelkada@gmail.com>
Co-authored-by: Younes Belkada <49240599+younesbelkada@users.noreply.github.com>
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

docs/source/en/_toctree.yml

@@ -399,6 +399,8 @@
         title: RoCBert
       - local: model_doc/roformer
         title: RoFormer
+      - local: model_doc/rwkv
+        title: RWKV
       - local: model_doc/splinter
         title: Splinter
       - local: model_doc/squeezebert