Change head files' structure

mmdet3d/models/__init__.py

 from .anchor_heads import *  # noqa: F401,F403
 from .backbones import *  # noqa: F401,F403
-from .bbox_heads import *  # noqa: F401,F403
 from .builder import (build_backbone, build_detector, build_fusion_layer,
                       build_head, build_loss, build_middle_encoder, build_neck,
                       build_roi_extractor, build_shared_head,
@@ -11,7 +10,7 @@ from .losses import *  # noqa: F401,F403
 from .middle_encoders import *  # noqa: F401,F403
 from .necks import *  # noqa: F401,F403
 from .registry import FUSION_LAYERS, MIDDLE_ENCODERS, VOXEL_ENCODERS
-from .roi_extractors import *  # noqa: F401,F403
+from .roi_heads import *  # noqa: F401,F403
 from .voxel_encoders import *  # noqa: F401,F403
 
 __all__ = [

mmdet3d/models/bbox_heads/__init__.py

-from mmdet.models.bbox_heads import (BBoxHead, ConvFCBBoxHead,
-                                     DoubleConvFCBBoxHead, Shared2FCBBoxHead,
-                                     Shared4Conv1FCBBoxHead)
-
-__all__ = [
-    'BBoxHead', 'ConvFCBBoxHead', 'Shared2FCBBoxHead',
-    'Shared4Conv1FCBBoxHead', 'DoubleConvFCBBoxHead'
-]

mmdet3d/models/roi_extractors/__init__.py

-from mmdet.models.roi_extractors.single_level import SingleRoIExtractor
-
-__all__ = ['SingleRoIExtractor']