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>

tests/models/longformer/test_modeling_longformer.py

@@ -34,8 +34,8 @@ if is_torch_available():
         LongformerForSequenceClassification,
         LongformerForTokenClassification,
         LongformerModel,
-        LongformerSelfAttention,
     )
+    from transformers.models.longformer.modeling_longformer import LongformerSelfAttention
 
 
 class LongformerModelTester:

tests/models/longformer/test_modeling_tf_longformer.py

@@ -37,8 +37,8 @@ if is_tf_available():
         TFLongformerForSequenceClassification,
         TFLongformerForTokenClassification,
         TFLongformerModel,
-        TFLongformerSelfAttention,
     )
+    from transformers.models.longformer.modeling_tf_longformer import TFLongformerSelfAttention
     from transformers.tf_utils import shape_list