fix problem mentioned in traveller59#49.

Author: traveller59

second/pytorch/builder/box_coder_builder.py

@@ -19,7 +19,7 @@ def build(box_coder_config):
     """
     box_coder_type = box_coder_config.WhichOneof('box_coder')
     if box_coder_type == 'ground_box3d_coder':
-        cfg = box_coder_config.bev_box_coder
+        cfg = box_coder_config.ground_box3d_coder
         return GroundBox3dCoderTorch(cfg.linear_dim, cfg.encode_angle_vector)
     elif box_coder_type == 'bev_box_coder':
         cfg = box_coder_config.bev_box_coder