fix masked_conv cuda runtime error when mask is all zero (#779)

* fix mask conv import error

* fix masked_conv cuda runtime error when mask is all zero

mmdet/ops/masked_conv/functions/masked_conv.py

@@ -30,6 +30,8 @@ class MaskedConv2dFunction(Function):
             math.floor((features.size(3) + 2 * pad_w -
                         (kernel_h - 1) - 1) / stride_w + 1))
         mask_inds = torch.nonzero(mask[0] > 0)
+        output = features.new_zeros(batch_size, out_channel, out_h, out_w)
+        if mask_inds.numel() > 0:
             mask_h_idx = mask_inds[:, 0].contiguous()
             mask_w_idx = mask_inds[:, 1].contiguous()
             data_col = features.new_zeros(in_channel * kernel_h * kernel_w,
@@ -41,7 +43,6 @@ class MaskedConv2dFunction(Function):
 
             masked_output = torch.addmm(1, bias[:, None], 1,
                                         weight.view(out_channel, -1), data_col)
-        output = features.new_zeros(batch_size, out_channel, out_h, out_w)
             masked_conv2d_cuda.masked_col2im_forward(masked_output, mask_h_idx,
                                                      mask_w_idx, out_h, out_w,
                                                      out_channel, output)