Sampler (#1328)

* sampler

* check for dataloader type

* check for dataloader type

* fixed sampler cases

pytorch_lightning/trainer/data_loading.py

@@ -79,11 +79,11 @@ class TrainerDataLoadingMixin(ABC):
         if not isinstance(dataloader, DataLoader):
             return dataloader
 
-        # don't add sampler when user gives one
-        if dataloader.sampler is not None:
-            return dataloader
+        need_dist_sampler = self.use_ddp or self.use_ddp2 or self.use_tpu
+        no_sampler_added = dataloader.sampler is None
+
+        if need_dist_sampler and no_sampler_added:
 
-        if self.use_ddp or self.use_ddp2 or self.use_tpu:
             dl_args = {
                 'dataset': dataloader.dataset,
                 'batch_size': dataloader.batch_size,