Fix nn.DataParallel compatibility in PyTorch 1.5 (#4300)

* Test case for #3936

* multigpu tests pass on pytorch 1.4.0

* Fixup

* multigpu tests pass on pytorch 1.5.0

* Update src/transformers/modeling_utils.py

* Update src/transformers/modeling_utils.py

* rename multigpu to require_multigpu

* mode doc

src/transformers/modeling_utils.py

@@ -17,7 +17,7 @@
 import inspect
 import logging
 import os
-from typing import Callable, Tuple
+from typing import Callable, List, Tuple
 
 import torch
 from torch import Tensor, device, dtype, nn
@@ -110,11 +110,33 @@ class ModuleUtilsMixin:
 
     @property
     def device(self) -> device:
-        return next(self.parameters()).device
+        try:
+            return next(self.parameters()).device
+        except StopIteration:
+            # For nn.DataParallel compatibility in PyTorch 1.5
+
+            def find_tensor_attributes(module: nn.Module) -> List[Tuple[str, Tensor]]:
+                tuples = [(k, v) for k, v in module.__dict__.items() if torch.is_tensor(v)]
+                return tuples
+
+            gen = self._named_members(get_members_fn=find_tensor_attributes)
+            first_tuple = next(gen)
+            return first_tuple[1].device
 
     @property
     def dtype(self) -> dtype:
-        return next(self.parameters()).dtype
+        try:
+            return next(self.parameters()).dtype
+        except StopIteration:
+            # For nn.DataParallel compatibility in PyTorch 1.5
+
+            def find_tensor_attributes(module: nn.Module) -> List[Tuple[str, Tensor]]:
+                tuples = [(k, v) for k, v in module.__dict__.items() if torch.is_tensor(v)]
+                return tuples
+
+            gen = self._named_members(get_members_fn=find_tensor_attributes)
+            first_tuple = next(gen)
+            return first_tuple[1].dtype
 
     def invert_attention_mask(self, encoder_attention_mask: Tensor) -> Tensor:
         """type: torch.Tensor -> torch.Tensor"""