fix (dpool): directly return empty if rois's length is 0 (#2099)

* fix (dpool): directly return empty if rois's length is 0

* fix (dpool): also skip empty when no_trans=True

* reformat (test/test_dpool.py): reformat to pass flake8

* fix (test_dpool): skip gpu test when GPU is not avaliable

* rm (test_dpool.py): rm test_dpool since it is not very necessary in CI

mmdet/ops/dcn/deform_pool.py

@@ -148,6 +148,10 @@ def __init__(self,
 
     def forward(self, data, rois):
         assert data.size(1) == self.out_channels
+        n = rois.shape[0]
+        if n == 0:
+            return data.new_empty(n, self.out_channels, self.out_size[0],
+                                  self.out_size[1])
         if self.no_trans:
             offset = data.new_empty(0)
             return deform_roi_pooling(data, rois, offset, self.spatial_scale,
@@ -156,7 +160,6 @@ def forward(self, data, rois):
                                       self.part_size, self.sample_per_part,
                                       self.trans_std)
         else:
-            n = rois.shape[0]
             offset = data.new_empty(0)
             x = deform_roi_pooling(data, rois, offset, self.spatial_scale,
                                    self.out_size, self.out_channels, True,
@@ -228,6 +231,10 @@ def __init__(self,
 
     def forward(self, data, rois):
         assert data.size(1) == self.out_channels
+        n = rois.shape[0]
+        if n == 0:
+            return data.new_empty(n, self.out_channels, self.out_size[0],
+                                  self.out_size[1])
         if self.no_trans:
             offset = data.new_empty(0)
             return deform_roi_pooling(data, rois, offset, self.spatial_scale,
@@ -236,7 +243,6 @@ def forward(self, data, rois):
                                       self.part_size, self.sample_per_part,
                                       self.trans_std)
         else:
-            n = rois.shape[0]
             offset = data.new_empty(0)
             x = deform_roi_pooling(data, rois, offset, self.spatial_scale,
                                    self.out_size, self.out_channels, True,