Add type hint in mmcv/ops/points_in_boxes.py (#2007)

* add type hint in mmcv/ops/points_in_boxes.py

* fix lint

mmcv/ops/points_in_boxes.py

 import torch
+from torch import Tensor
 
 from ..utils import ext_loader
 
@@ -8,7 +9,7 @@ ext_module = ext_loader.load_ext('_ext', [
 ])
 
 
-def points_in_boxes_part(points, boxes):
+def points_in_boxes_part(points: Tensor, boxes: Tensor) -> Tensor:
     """Find the box in which each point is (CUDA).
 
     Args:
@@ -56,7 +57,7 @@ def points_in_boxes_part(points, boxes):
     return box_idxs_of_pts
 
 
-def points_in_boxes_cpu(points, boxes):
+def points_in_boxes_cpu(points: Tensor, boxes: Tensor) -> Tensor:
     """Find all boxes in which each point is (CPU). The CPU version of
     :meth:`points_in_boxes_all`.
 
@@ -94,7 +95,7 @@ def points_in_boxes_cpu(points, boxes):
     return point_indices
 
 
-def points_in_boxes_all(points, boxes):
+def points_in_boxes_all(points: Tensor, boxes: Tensor) -> Tensor:
     """Find all boxes in which each point is (CUDA).
 
     Args: