[Bart] Fix: put dummy_inputs on correct device (#3398)

* Dummy inputs to model.device

* Move self.device to ModuleUtilsMixin

src/transformers/modeling_utils.py

@@ -108,6 +108,10 @@ class ModuleUtilsMixin:
             module.mem_rss_post_forward = 0
             module.mem_rss_pre_forward = 0
 
+    @property
+    def device(self):
+        return next(self.parameters()).device
+
 
 class PreTrainedModel(nn.Module, ModuleUtilsMixin):
     r""" Base class for all models.