Adding _tie_weights() to prediction heads to support low_cpu_mem_usage=True (#29024)

* Adding _tie_weights() to prediction heads to support low_cpu_mem_usage=True

* Testing for the non-safe-tensors case, since the default is safe-tensors already

* Running fixup/fix-copies

* Adding accelerate annotations to tests

tests/models/musicgen/test_modeling_musicgen.py

@@ -1273,6 +1273,18 @@ class MusicgenTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMixin,
     def test_tied_weights_keys(self):
         pass
 
+    @unittest.skip("No support for low_cpu_mem_usage=True.")
+    def test_save_load_low_cpu_mem_usage(self):
+        pass
+
+    @unittest.skip("No support for low_cpu_mem_usage=True.")
+    def test_save_load_low_cpu_mem_usage_checkpoints(self):
+        pass
+
+    @unittest.skip("No support for low_cpu_mem_usage=True.")
+    def test_save_load_low_cpu_mem_usage_no_safetensors(self):
+        pass
+
     # override since changing `output_hidden_states` / `output_attentions` from the top-level model config won't work
     def test_retain_grad_hidden_states_attentions(self):
         config, inputs_dict = self.model_tester.prepare_config_and_inputs_for_common()