Encoder-decoder models: move embedding scale to nn.Module (#30410)

* move scaling to nn.Module

* let the test be here for now (need to fix)

* failing tests

* last failing models

* Revert commit 4c14817f38

* clean-up

* oops forgot

* codestyle

* raise NotImplemented when possible

* Update tests/test_modeling_common.py

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

* skip tests in respective modeling files

---------

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

tests/models/idefics2/test_modeling_idefics2.py

@@ -180,6 +180,10 @@ class Idefics2ModelTest(ModelTesterMixin, unittest.TestCase):
     def test_inputs_embeds():
         pass
 
+    @unittest.skip("input_embeds cannot be passed in without input_ids")
+    def test_inputs_embeds_matches_input_ids(self):
+        pass
+
     @unittest.skip("Model does not support padding right")
     def test_flash_attn_2_generate_padding_right(self):
         pass