fix scatter in pytorch18 (#882)

* fix scatter in pytorch18

* remove blanks

mmcv/parallel/distributed.py

@@ -18,6 +18,11 @@ class MMDistributedDataParallel(DistributedDataParallel):
     - It implement two APIs ``train_step()`` and ``val_step()``.
     """
 
+    def to_kwargs(self, inputs, kwargs, device_id):
+        # Use `self.to_kwargs` instead of `self.scatter` in pytorch1.8
+        # to move all tensors to device_id
+        return scatter_kwargs(inputs, kwargs, [device_id], dim=self.dim)
+
     def scatter(self, inputs, kwargs, device_ids):
         return scatter_kwargs(inputs, kwargs, device_ids, dim=self.dim)