Add a main_input_name attribute to all models (#14803)

* Add a main_input_name attribute to all models

* Fix tests

* Wtf Vs Code?

* Update src/transformers/models/imagegpt/modeling_imagegpt.py

Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

* Style

* Fix copies

Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

tests/test_modeling_flax_common.py

@@ -778,6 +778,13 @@ class FlaxModelTesterMixin:
             for name, type_ in types.items():
                 self.assertEqual(type_, jnp.bfloat16, msg=f"param {name} is not in bf16.")
 
+    def test_model_main_input_name(self):
+        for model_class in self.all_model_classes:
+            model_signature = inspect.signature(getattr(model_class, "__call__"))
+            # The main input is the name of the argument after `self`
+            observed_main_input_name = list(model_signature.parameters.keys())[1]
+            self.assertEqual(model_class.main_input_name, observed_main_input_name)
+
     def test_headmasking(self):
         if not self.test_head_masking:
             return