Fix mmcv-dataparallel (#497)

* Fix mmcv-dataparallel

* Fix (parallel): fix CPU inference with MMDataParallel

* Update docstrings

* Doc (parallel): refine docstrings

* Fix (parallel): fix missing changes of train/val step function

* resolve comments

* Fix (data_parallel): fix bug when single gpu test return None

mmcv/parallel/_functions.py

@@ -19,8 +19,13 @@ def scatter(input, devices, streams=None):
         output = input.contiguous()
         # TODO: copy to a pinned buffer first (if copying from CPU)
         stream = streams[0] if output.numel() > 0 else None
-        with torch.cuda.device(devices[0]), torch.cuda.stream(stream):
-            output = output.cuda(devices[0], non_blocking=True)
+        if devices != [-1]:
+            with torch.cuda.device(devices[0]), torch.cuda.stream(stream):
+                output = output.cuda(devices[0], non_blocking=True)
+        else:
+            # unsquzee the first dimension thus the tensor's shape is the
+            # same as those scattered with GPU.
+            output = output.unsqueeze(0)
         return output
     else:
         raise Exception(f'Unknown type {type(input)}.')
@@ -62,7 +67,7 @@ class Scatter:
     def forward(target_gpus, input):
         input_device = get_input_device(input)
         streams = None
-        if input_device == -1:
+        if input_device == -1 and target_gpus != [-1]:
             # Perform CPU to GPU copies in a background stream
             streams = [_get_stream(device) for device in target_gpus]
 

mmcv/parallel/data_parallel.py

@@ -7,12 +7,48 @@ from .scatter_gather import scatter_kwargs
 
 
 class MMDataParallel(DataParallel):
+    """The DataParallel module that supports DataContainer.
+
+    MMDataParallel has two main differences with PyTorch DataParallel:
+
+    - It supports a custom type :class:`DataContainer` which allows more
+      flexible control of input data during both GPU and CPU inference.
+    - It implement two more APIs ``train_step()`` and ``val_step()``.
+
+    Args:
+        module (:class:`nn.Module`): Module to be encapsulated.
+        device_ids (list[int]): Device IDS of modules to be scattered to.
+            Defaults to None when GPU is not available.
+        output_device (str | int): Device ID for output. Defaults to None.
+        dim (int): Dimension used to scatter the data. Defaults to 0.
+    """
+
+    def __init__(self, *args, dim=0, **kwargs):
+        super(MMDataParallel, self).__init__(*args, dim=dim, **kwargs)
+        self.dim = dim
+
+    def forward(self, *inputs, **kwargs):
+        """Override the original forward function.
+
+        The main difference lies in the CPU inference where the datas in
+        :class:`DataContainers` will still be gathered.
+        """
+        if not self.device_ids:
+            # We add the following line thus the module could gather and
+            # convert data containers as those in GPU inference
+            inputs, kwargs = self.scatter(inputs, kwargs, [-1])
+            return self.module(*inputs[0], **kwargs[0])
+        else:
+            return super().forward(*inputs, **kwargs)
 
     def scatter(self, inputs, kwargs, device_ids):
         return scatter_kwargs(inputs, kwargs, device_ids, dim=self.dim)
 
     def train_step(self, *inputs, **kwargs):
         if not self.device_ids:
+            # We add the following line thus the module could gather and
+            # convert data containers as those in GPU inference
+            inputs, kwargs = self.scatter(inputs, kwargs, [-1])
             return self.module.train_step(*inputs, **kwargs)
 
         assert len(self.device_ids) == 1, \
@@ -32,6 +68,9 @@ class MMDataParallel(DataParallel):
 
     def val_step(self, *inputs, **kwargs):
         if not self.device_ids:
+            # We add the following line thus the module could gather and
+            # convert data containers as those in GPU inference
+            inputs, kwargs = self.scatter(inputs, kwargs, [-1])
             return self.module.val_step(*inputs, **kwargs)
 
         assert len(self.device_ids) == 1, \

mmcv/parallel/scatter_gather.py

@@ -15,7 +15,11 @@ def scatter(inputs, target_gpus, dim=0):
 
     def scatter_map(obj):
         if isinstance(obj, torch.Tensor):
-            return OrigScatter.apply(target_gpus, None, dim, obj)
+            if target_gpus != [-1]:
+                return OrigScatter.apply(target_gpus, None, dim, obj)
+            else:
+                # for CPU inference we use self-implemented scatter
+                return Scatter.forward(target_gpus, obj)
         if isinstance(obj, DataContainer):
             if obj.cpu_only:
                 return obj.data