fix pipeline forward return tensors (#176)

colossalai/engine/schedule/_pipeline_schedule.py

 #!/usr/bin/env python
 # -*- encoding: utf-8 -*-
 
-from typing import List, Tuple, Union, Callable
 import inspect
-import torch.cuda
+from typing import Callable, List, Tuple, Union
 
 import colossalai.communication as comm
+import torch.cuda
+from colossalai.amp.naive_amp import NaiveAMPModel
 from colossalai.context.parallel_mode import ParallelMode
 from colossalai.core import global_context as gpc
-from colossalai.amp.naive_amp import NaiveAMPModel
+from colossalai.logging import get_dist_logger
+from colossalai.utils import switch_virtual_pipeline_parallel_rank
 from colossalai.utils.cuda import get_current_device
 from colossalai.zero import (ZeroRedundancyOptimizer_Level_2,
                              ZeroRedundancyOptimizer_Level_3)
-from colossalai.utils import switch_virtual_pipeline_parallel_rank
-from colossalai.logging import get_dist_logger
+
 from ._base_schedule import BaseSchedule
 
 
@@ -151,7 +152,7 @@ class PipelineSchedule(BaseSchedule):
 
         if gpc.is_last_rank(ParallelMode.PIPELINE):
             if return_output_label:
-                return_tensors.append(tuple((output_tensor, label)))
+                return_tensors.append((output_tensor, label))
             if accum_loss is not None:
                 loss_reduced = self._call_engine_criterion(engine, output_tensor, label) / self.num_microbatches
                 accum_loss.add_(loss_reduced.detach())
@@ -414,7 +415,7 @@ class InterleavedPipelineSchedule(PipelineSchedule):
 
         if gpc.is_pipeline_last_stage():
             if return_output_label:
-                return_tensors.append(tuple(output_tensor, label))
+                return_tensors.append((output_tensor, label))
             if accum_loss is not None:
                 loss_reduced = self._call_engine_criterion(engine, output_tensor, label) / self.num_microbatches
                 accum_loss.add_(loss_reduced.detach())