fix nn.DataParallel compatibility with PyTorch 1.5 (#7671)

The same type of errors as in https://github.com/huggingface/transformers/pull/4300

src/transformers/modeling_lxmert.py

@@ -964,7 +964,7 @@ class LxmertModel(LxmertPreTrainedModel):
         # Process the visual attention mask
         if visual_attention_mask is not None:
             extended_visual_attention_mask = visual_attention_mask.unsqueeze(1).unsqueeze(2)
-            extended_visual_attention_mask = extended_visual_attention_mask.to(dtype=next(self.parameters()).dtype)
+            extended_visual_attention_mask = extended_visual_attention_mask.to(dtype=self.dtype)
             extended_visual_attention_mask = (1.0 - extended_visual_attention_mask) * -10000.0
         else:
             extended_visual_attention_mask = None