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/musicgen/test_modeling_musicgen.py

@@ -265,6 +265,10 @@ class MusicgenDecoderTest(ModelTesterMixin, GenerationTesterMixin, PipelineTeste
             lm_heads = model.get_output_embeddings()
             self.assertTrue(lm_heads is None or isinstance(lm_heads[0], torch.nn.Linear))
 
+    @unittest.skip(reason="MusicGen does not use inputs_embeds")
+    def test_inputs_embeds_matches_input_ids(self):
+        pass
+
     # skip as this model doesn't support all arguments tested
     def test_model_outputs_equivalence(self):
         pass