[Fix] minor bug for single output in _DistributedDataParallel (#177)

## Title

- Fix minor bug for single output in _DistributedDataParallel

## Description

- This PR addresses a minor bug in the `_DistributedDataParallel` class
when handling single output tensors. The changes include:

1. Update the `forward` method in `_DistributedDataParallel` to
correctly handle single output tensors.
2. Add new test cases in
`tests_deprecated/torch/nn/parallel/data_parallel/data_parallel.py` to
ensure the correct behavior for models with various output types (single
tensor, multiple tensors, and dictionary of tensors).

These updates will ensure that the `_DistributedDataParallel` class
works correctly with various output types, providing a more robust
solution for users.

## Linked Issues

- N/A

oslo/torch/nn/parallel/data_parallel/data_parallel.py

@@ -114,9 +114,12 @@ def forward(self, *args, **kwargs):
                 }
             )
 
-        if isinstance(inputs, torch.Tensor):
+        single_output = isinstance(inputs, torch.Tensor)
+        if single_output:
             inputs = (inputs,)
-        return _DistributedBackwardFunction.apply(self, *inputs)
+
+        outputs = _DistributedBackwardFunction.apply(self, *inputs)
+        return outputs[0] if single_output else outputs
 
     def _pre_backward(self):
         pass

tests_deprecated/torch/nn/parallel/data_parallel/data_parallel.py

@@ -0,0 +1,104 @@
+import os
+import torch
+import torch.nn as nn
+import torch.multiprocessing as mp
+
+from oslo.torch.utils import get_free_port
+from oslo.torch.distributed.parallel_context import ParallelContext
+from oslo.torch.nn.parallel.data_parallel.data_parallel import _DistributedDataParallel
+import copy
+
+import pytest
+
+skip_if_dist_unavailable = pytest.mark.skipif(
+    torch.cuda.device_count() < 2, reason="dist required"
+)
+
+
+class MlpModel(nn.Module):
+    def __init__(self):
+        super(MlpModel, self).__init__()
+        self.linear1 = nn.Linear(128, 256)
+        self.linear2 = nn.Linear(256, 512)
+
+    def forward(self, x):
+        x = self.linear1(x)
+        x = self.linear2(x)
+        return x
+
+
+class DictOutputModel(nn.Module):
+    def __init__(self):
+        super(DictOutputModel, self).__init__()
+        self.linear1 = nn.Linear(128, 256)
+        self.linear2 = nn.Linear(128, 512)
+
+    def forward(self, x):
+        x1 = self.linear1(x)
+        x2 = self.linear2(x)
+        return {"output1": x1, "output2": x2}
+
+
+class MultiOutputModel(nn.Module):
+    def __init__(self):
+        super(MultiOutputModel, self).__init__()
+        self.linear1 = nn.Linear(128, 256)
+        self.linear2 = nn.Linear(128, 512)
+
+    def forward(self, x):
+        x1 = self.linear1(x)
+        x2 = self.linear2(x)
+        return x1, x2
+
+
+def run_dist(rank, world_size, model_class):
+    os.environ["RANK"] = str(rank)
+    os.environ["LOCAL_RANK"] = str(rank)
+    parallel_context = ParallelContext.from_torch(data_parallel_size=world_size)
+
+    device = torch.device(f"cuda:{rank}" if torch.cuda.is_available() else "cpu")
+
+    model = model_class()
+    ddp_model = _DistributedDataParallel(
+        copy.deepcopy(model).to(device), parallel_context
+    )
+    ddp_model.parallelize()
+    model = model.to(device)
+
+    input_data = torch.randn(32, 128).to(device)
+
+    output_normal = model(input_data)
+
+    output_ddp = ddp_model(input_data)
+
+    if isinstance(output_normal, dict):
+        for key in output_normal:
+            assert torch.allclose(
+                output_normal[key], output_ddp[key], rtol=1e-03, atol=1e-03
+            ), f"Outputs do not match for key {key}!"
+    elif isinstance(output_normal, tuple):
+        for i, (out_n, out_f) in enumerate(zip(output_normal, output_ddp)):
+            assert torch.allclose(
+                out_n, out_f, rtol=1e-03, atol=1e-03
+            ), f"Outputs do not match for index {i}!"
+    else:
+        assert torch.allclose(
+            output_normal, output_ddp, rtol=1e-03, atol=1e-03
+        ), "Outputs do not match!"
+
+
+@skip_if_dist_unavailable
+def test_ddp():
+    world_size = 2
+    os.environ["WORLD_SIZE"] = str(world_size)
+    os.environ["LOCAL_WORLD_SIZE"] = str(world_size)
+    os.environ["MASTER_ADDR"] = "localhost"
+    os.environ["MASTER_PORT"] = str(get_free_port())
+
+    models_to_test = [MlpModel, DictOutputModel, MultiOutputModel]
+
+    for model_class in models_to_test:
+        print(f"Testing {model_class.__name__}")
+        mp.spawn(run_dist, args=(world_size, model_class), nprocs=world_size, join=True)
+
+    print("All tests passed!")