Import structure & first three model refactors (#31329)

* Import structure & first three model refactors

* Register -> Export. Export all in __all__. Sensible defaults according to filename.

* Apply most comments from Amy and some comments from Lucain

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

* Style

* Add comment

* Clearer .py management

* Raise if not in backend mapping

* More specific type

* More efficient listdir

* Misc fixes

---------

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

Makefile

@@ -53,7 +53,6 @@ quality:
 	@python -c "from transformers import *" || (echo '🚨 import failed, this means you introduced unprotected imports! 🚨'; exit 1)
 	ruff check $(check_dirs) setup.py conftest.py
 	ruff format --check $(check_dirs) setup.py conftest.py
-	python utils/custom_init_isort.py --check_only
 	python utils/sort_auto_mappings.py --check_only
 	python utils/check_doc_toc.py
 	python utils/check_docstrings.py --check_all
@@ -62,7 +61,6 @@ quality:
 # Format source code automatically and check is there are any problems left that need manual fixing
 
 extra_style_checks:
-	python utils/custom_init_isort.py
 	python utils/sort_auto_mappings.py
 	python utils/check_doc_toc.py --fix_and_overwrite