Add type hints in iou3d.py (#1989)

* add type hints in iou3d.py

* fix lint

mmcv/ops/iou3d.py

 # Copyright (c) OpenMMLab. All rights reserved.
 import warnings
+from typing import Optional
 
 import torch
+from torch import Tensor
 
 from ..utils import ext_loader
 
@@ -11,7 +13,7 @@ ext_module = ext_loader.load_ext('_ext', [
 ])
 
 
-def boxes_iou3d(boxes_a, boxes_b):
+def boxes_iou3d(boxes_a: Tensor, boxes_b: Tensor) -> Tensor:
     """Calculate boxes 3D IoU.
 
     Args:
@@ -30,7 +32,7 @@ def boxes_iou3d(boxes_a, boxes_b):
     return ans_iou
 
 
-def nms3d(boxes, scores, iou_threshold):
+def nms3d(boxes: Tensor, scores: Tensor, iou_threshold: float) -> Tensor:
     """3D NMS function GPU implementation (for BEV boxes).
 
     Args:
@@ -54,7 +56,8 @@ def nms3d(boxes, scores, iou_threshold):
     return keep
 
 
-def nms3d_normal(boxes, scores, iou_threshold):
+def nms3d_normal(boxes: Tensor, scores: Tensor,
+                 iou_threshold: float) -> Tensor:
     """Normal 3D NMS function GPU implementation. The overlap of two boxes for
     IoU calculation is defined as the exact overlapping area of the two boxes
     WITH their yaw angle set to 0.
@@ -79,7 +82,7 @@ def nms3d_normal(boxes, scores, iou_threshold):
     return order[keep[:num_out].cuda(boxes.device)].contiguous()
 
 
-def _xyxyr2xywhr(boxes):
+def _xyxyr2xywhr(boxes: Tensor) -> Tensor:
     """Convert [x1, y1, x2, y2, heading] box to [x, y, dx, dy, heading] box.
 
     Args:
@@ -119,7 +122,11 @@ def boxes_iou_bev(boxes_a, boxes_b):
     return box_iou_rotated(_xyxyr2xywhr(boxes_a), _xyxyr2xywhr(boxes_b))
 
 
-def nms_bev(boxes, scores, thresh, pre_max_size=None, post_max_size=None):
+def nms_bev(boxes: Tensor,
+            scores: Tensor,
+            thresh: float,
+            pre_max_size: Optional[int] = None,
+            post_max_size: Optional[int] = None) -> Tensor:
     """NMS function GPU implementation (for BEV boxes).
 
     The overlap of two
@@ -159,7 +166,7 @@ def nms_bev(boxes, scores, thresh, pre_max_size=None, post_max_size=None):
     return keep
 
 
-def nms_normal_bev(boxes, scores, thresh):
+def nms_normal_bev(boxes: Tensor, scores: Tensor, thresh: float) -> Tensor:
     """Normal NMS function GPU implementation (for BEV boxes).
 
     The overlap of