fix_mbart_tied_weights (#26422)

* fix_mbart_tied_weights

* add test

tests/models/mbart/test_modeling_mbart.py

@@ -327,6 +327,43 @@ class MBartModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMixi
         model.generate(input_ids, attention_mask=attention_mask)
         model.generate(num_beams=4, do_sample=True, early_stopping=False, num_return_sequences=3)
 
+    def test_ensure_weights_are_shared(self):
+        config, inputs_dict = self.model_tester.prepare_config_and_inputs()
+
+        config.tie_word_embeddings = True
+        model = MBartForConditionalGeneration(config)
+
+        # MBart shares four weights.
+        # Not an issue to not have these correctly tied for torch.load, but it is an issue for safetensors.
+        self.assertEqual(
+            len(
+                {
+                    model.get_output_embeddings().weight.data_ptr(),
+                    model.get_input_embeddings().weight.data_ptr(),
+                    model.base_model.decoder.embed_tokens.weight.data_ptr(),
+                    model.base_model.encoder.embed_tokens.weight.data_ptr(),
+                }
+            ),
+            1,
+        )
+
+        config.tie_word_embeddings = False
+        model = MBartForConditionalGeneration(config)
+
+        # MBart shares four weights.
+        # Not an issue to not have these correctly tied for torch.load, but it is an issue for safetensors.
+        self.assertEqual(
+            len(
+                {
+                    model.get_output_embeddings().weight.data_ptr(),
+                    model.get_input_embeddings().weight.data_ptr(),
+                    model.base_model.decoder.embed_tokens.weight.data_ptr(),
+                    model.base_model.encoder.embed_tokens.weight.data_ptr(),
+                }
+            ),
+            2,
+        )
+
 
 def assert_tensors_close(a, b, atol=1e-12, prefix=""):
     """If tensors have different shapes, different values or a and b are not both tensors, raise a nice Assertion error."""