added set_epoch for distributed sampler, fix for #224 (#225)

pytorch_lightning/trainer/trainer.py

@@ -929,6 +929,10 @@ class Trainer(TrainerIO):
     def __train(self):
         # run all epochs
         for epoch_nb in range(self.current_epoch, self.max_nb_epochs):
+            # set seed for distributed sampler (enables shuffling for each epoch)
+            if self.use_ddp:
+                self.tng_dataloader.sampler.set_epoch(epoch_nb)
+
             # get model
             model = self.__get_model()