[Llama2] Add support for Llama 2 (#24891)

* add llama

* add other readmes

* update padding id in readme

* add link to paper

* fix paths and tokenizer

* more nits

* styling

* fit operation in 2 lines when possible

* nits

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* add form

* update reademe

* update readme, we don't have a default pad token

* update test and tokenization

* LLaMA instead of Llama

* nits

* add expected text

* add greeedy output

* styling

* Update src/transformers/models/llama/modeling_llama.py

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* sequential device map

* skip relevant changes

---------

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

docs/source/en/_toctree.yml

@@ -335,6 +335,8 @@
         title: LED
       - local: model_doc/llama
         title: LLaMA
+      - local: model_doc/llama2
+        title: Llama2
       - local: model_doc/longformer
         title: Longformer
       - local: model_doc/longt5