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>

README_ja.md

@@ -444,6 +444,7 @@ Flax、PyTorch、TensorFlowをcondaでインストールする方法は、それ
 1. **[RoBERTa-PreLayerNorm](https://huggingface.co/docs/transformers/model_doc/roberta-prelayernorm)** (Facebook から) Myle Ott, Sergey Edunov, Alexei Baevski, Angela Fan, Sam Gross, Nathan Ng, David Grangier, Michael Auli から公開された研究論文: [fairseq: A Fast, Extensible Toolkit for Sequence Modeling](https://arxiv.org/abs/1904.01038)
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+1. **[RWKV](https://huggingface.co/docs/transformers/main/model_doc/rwkv)** (Bo Peng から) Bo Peng. から公開された研究論文 [this repo](https://github.com/BlinkDL/RWKV-LM)
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 1. **[Segment Anything](https://huggingface.co/docs/transformers/main/model_doc/sam)** (Meta AI から) Alexander Kirillov, Eric Mintun, Nikhila Ravi, Hanzi Mao, Chloe Rolland, Laura Gustafson, Tete Xiao, Spencer Whitehead, Alex Berg, Wan-Yen Lo, Piotr Dollar, Ross Girshick. から公開された研究論文 [Segment Anything](https://arxiv.org/pdf/2304.02643v1.pdf)
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