allow to configure out the mask_head of the HTC algorithm (open-mmlab#5389)

The HTC algorithm has an interleaved execution of bbox_heads and
mask_heads. For Object-Detection we only need the bbox_head.
The design of the HTC algorithms allows to remove the mask_head
while the bbox_head keeps working.

Co-authored-by: Maximilian Otten <[email protected]>

Author: mxotten

mmdet/models/roi_heads/htc_roi_head.py

@@ -25,7 +25,7 @@ def __init__(self,
                  **kwargs):
         super(HybridTaskCascadeRoIHead,
               self).__init__(num_stages, stage_loss_weights, **kwargs)
-        assert self.with_bbox and self.with_mask
+        assert self.with_bbox
         assert not self.with_shared_head  # shared head is not supported
 
         if semantic_head is not None: