[WIP] Adding GPT-NeoX-20B (#16659)

* initial

* first try

* working 20B

* 20B tokenizers

* Docs

* Import fixes for missing classes

* Update docs, fixup

* black formatting

* isort

* flake

* dummy objects

* documentation

* Documentation yml

* more docs

* tweaks for tests

* tokenization auto

* fix neox tests

* test

* test

* einsum

* address PR feedback

* Documentation

* Update README.md

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

* Update src/transformers/models/gpt_neox/__init__.py

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

* Update src/transformers/models/gpt_neox/configuration_gpt_neox.py

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

* Apply suggestions from code review

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

* Remove undefined LaTeX syntax

* Update to full url to avoid confusion about if that's supposed to refer to the Hub

* fix auto

* move tests

* documentation fix

* more doc fixes

* test refactor

* fix import

* fix import

* fix import

* fix import

* fix import

* style fixes

* More modeling fixes

Co-authored-by: Jason Phang <zp489@gr057.hpc.nyu.edu>
Co-authored-by: Stella Biderman <stellabiderman@gmail.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

docs/source/en/_toctree.yml

@@ -286,6 +286,8 @@
       title: GPT-J
     - local: model_doc/gpt_neo
       title: GPT Neo
+    - local: model_doc/gpt_neox
+      title: GPT NeoX
     - local: model_doc/hubert
       title: Hubert
     - local: model_doc/perceiver